View source for Dark matter

{{Short description|Concept in cosmology}}
{{hatnote group|
{{Other uses|Dark Matter (disambiguation)}}
{{Distinguish|Antimatter|Dark energy}}
}}
{{Use dmy dates|date=September 2019}}
<!-- Before changing percentages, please note that 85% here refers to that of matter /excluding/ dark energy: (Dark matter 26.8%, Dark energy 68.3%, Ordinary matter 4.9%, Total 95.1% is Dark matter & Dark energy)-->
{{unsolved|physics|What is dark matter? How was it generated?}}
{{Physical cosmology|comp/struct}}

In [[astronomy]], '''dark matter''' is a hypothetical form of [[matter]] that does not interact with [[light]] or other [[electromagnetic radiation]]. Dark matter is implied by [[gravity|gravitational]] effects which cannot be explained by [[general relativity]] unless more matter is present than can be observed. Such effects occur in the context of [[Galaxy formation and evolution|formation and evolution of galaxies]],<ref name="Siegfried">{{Cite news |last=Siegfried |first=T. |date=5 July 1999 |title=Hidden space dimensions may permit parallel universes, explain cosmic mysteries |url=http://www.physics.ucdavis.edu/~kaloper/siegfr.txt |work=The Dallas Morning News}}</ref> [[gravitational lensing]],<ref name="Trimble 1987">{{cite journal |last=Trimble |first=V. |date=1987 |title=Existence and nature of dark matter in the universe |journal=[[Annual Review of Astronomy and Astrophysics]] |volume=25 |pages=425–472 |bibcode=1987ARA&A..25..425T |doi=10.1146/annurev.aa.25.090187.002233 |s2cid=123199266 |url=https://cloudfront.escholarship.org/dist/prd/content/qt2hz008rs/qt2hz008rs.pdf |archive-url=https://web.archive.org/web/20180718231719/https://cloudfront.escholarship.org/dist/prd/content/qt2hz008rs/qt2hz008rs.pdf |archive-date=2018-07-18 |url-status=live|issn=0066-4146 }}</ref> the [[observable universe]]'s current structure, mass position in [[galactic collision]]s,<ref>{{Cite web |url=https://arstechnica.com/science/2017/02/a-history-of-dark-matter |title=A history of dark matter |year=2017}}</ref> the motion of galaxies within [[galaxy cluster]]s, and [[cosmic microwave background]] [[Anisotropy|anisotropies]].

In the standard [[lambda-CDM model]] of [[cosmology]], the [[mass–energy equivalence|mass–energy]] content of the universe is 5% ordinary matter, 26.8% dark matter, and 68.2% a form of energy known as [[dark energy]].<ref name="NASA Planck Mission">{{cite web |url=http://www.nasa.gov/mission_pages/planck/news/planck20130321.html |title=Planck Mission Brings Universe into Sharp Focus |website=NASA Mission Pages |date=21 March 2013 |access-date=1 May 2016 |archive-date=12 November 2020 |archive-url=https://web.archive.org/web/20201112001039/http://www.nasa.gov/mission_pages/planck/news/planck20130321.html |url-status=dead }}</ref><ref name="NASA Science Dark Matter">{{cite web |url=https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/ |title=Dark Energy, Dark Matter |website=NASA Science: Astrophysics |date=5 June 2015}}</ref><ref name="planck_overview">{{cite journal |first1=P. A. R. |last1=Ade |first2=N. |last2=Aghanim |author2-link=Nabila Aghanim|first3=C. |last3=Armitage-Caplan |collaboration=Planck Collaboration |title=Planck 2013 results. I. Overview of products and scientific results – Table&nbsp;9 |journal=[[Astronomy and Astrophysics]] |volume=1303 |page=5062 |url=http://www.cosmos.esa.int/web/planck/publications |date=22 March 2013 |arxiv=1303.5062 |bibcode=2014A&A...571A...1P |doi=10.1051/0004-6361/201321529 |s2cid=218716838 |display-authors=etal}}</ref><ref name="wmap7parameters(a)">{{cite web |title=First Planck results: the Universe is still weird and interesting |url=https://arstechnica.com/science/2013/03/first-planck-results-the-universe-is-still-weird-and-interesting/ |author=Francis, Matthew |date=22 March 2013 |website=Ars Technica}}</ref> Thus, dark matter constitutes 85%<ref group="lower-alpha">Since dark energy does not count as matter, this is {{nowrap|{{sfrac|26.8|4.9 + 26.8}} {{=}} 0.845}}.</ref> of the total mass, while dark energy and dark matter constitute 95% of the total mass–energy content.<ref name="planckcam">{{cite web |url=http://www.cam.ac.uk/research/news/planck-captures-portrait-of-the-young-universe-revealing-earliest-light |title=Planck captures portrait of the young Universe, revealing earliest light |date=21 March 2013 |publisher=University of Cambridge |access-date=21 March 2013}}</ref><ref name="DarkMatter">{{cite book |first=Sean |last=Carroll |year=2007 |publisher=The Teaching Company |title=Dark Matter, Dark Energy: The dark side of the universe |at=Guidebook Part&nbsp;2 p.&nbsp;46 <!-- access-date=7 October 2013 --> |quote=...&nbsp;dark matter: An invisible, essentially collisionless component of matter that makes up about 25&nbsp;percent of the energy density of the universe ... it's a different kind of particle... something not yet observed in the laboratory&nbsp;...}}</ref><ref>{{cite magazine |title=Dark matter |magazine=National Geographic Magazine |department=Hidden cosmos |url=http://ngm.nationalgeographic.com/2015/01/hidden-cosmos/ferris-text |archive-url=https://web.archive.org/web/20141225013843/http://ngm.nationalgeographic.com/2015/01/hidden-cosmos/ferris-text |url-status=dead |archive-date=25 December 2014 |access-date=10 June 2015 |first=Timothy |last=Ferris |date=January 2015}}</ref><ref name="wmap7parameters">{{cite journal |last1=Jarosik |first1=N. |display-authors=etal |date=2011 |title=Seven-year Wilson microwave anisotropy probe (WMAP) observations: Sky maps, systematic errors, and basic results |journal=[[Astrophysical Journal Supplement]] |volume=192 |issue=2 |page=14 |arxiv=1001.4744 |bibcode=2011ApJS..192...14J |doi=10.1088/0067-0049/192/2/14|s2cid=46171526 }}</ref>

Dark matter is not known to interact with ordinary [[Baryonic matter#Baryonic matter|baryonic matter]] and [[radiation]] except through gravity, making it difficult to detect in the laboratory. The most prevalent explanation is that dark matter is some as-yet-undiscovered [[subatomic particle]], such as either [[weakly interacting massive particle]]s (WIMPs) or [[axion]]s.<ref name="ars lensing">{{cite news |last1=Timmer |first1=John |date=21 April 2023 |title=No WIMPS! Heavy particles don't explain gravitational lensing oddities |language=en-us |work=Ars Technica |url=https://arstechnica.com/science/2023/04/gravitational-lensing-may-point-to-lighter-dark-matter-candidate/ |access-date=21 June 2023}}</ref> The other main possibility is that dark matter is composed of [[primordial black hole]]s.<ref name="Carr24">{{cite journal |last1=Carr |first1=B. J. |last2=Clesse |first2=S. |last3=García-Bellido |first3=J. |last4=Hawkins |first4=M. R. S. |last5=Kühnel |first5=F. |title=Observational evidence for primordial black holes: A positivist perspective |journal=Physics Reports |date=26 February 2024 |volume=1054 |pages=1–68 |doi=10.1016/j.physrep.2023.11.005 |url=https://www.sciencedirect.com/science/article/pii/S0370157323003976 |issn=0370-1573|arxiv=2306.03903 |bibcode=2024PhR..1054....1C }} See Figure 39.</ref><ref name="Bird">{{cite journal |last1=Bird |first1=Simeon |last2=Albert |first2=Andrea |last3=Dawson |first3=Will |last4=Ali-Haïmoud |first4=Yacine |last5=Coogan |first5=Adam |last6=Drlica-Wagner |first6=Alex |last7=Feng |first7=Qi |last8=Inman |first8=Derek |last9=Inomata |first9=Keisuke |last10=Kovetz |first10=Ely |last11=Kusenko |first11=Alexander |last12=Lehmann |first12=Benjamin V. |last13=Muñoz |first13=Julian B. |last14=Singh |first14=Rajeev |last15=Takhistov |first15=Volodymyr |last16=Tsai |first16=Yu-Dai |title=Primordial black hole dark matter |journal=Physics of the Dark Universe |date=1 August 2023 |volume=41 |page=101231 |doi=10.1016/j.dark.2023.101231 |arxiv=2203.08967 |s2cid=247518939 |issn=2212-6864}}</ref><ref name="Carr" />

Dark matter is classified as "cold", "warm", or "hot" according to [[velocity]] (more precisely, its [[free streaming]] length). Recent models have favored a [[cold dark matter]] scenario, in which [[Structure formation|structures emerge]] by the gradual accumulation of particles.

Although the astrophysics community generally accepts dark matter's existence,<ref>{{cite journal|url=https://www.scientificamerican.com/article/is-dark-matter-real/|title=Is dark matter real?|date=August 2018|last1=Hossenfelder |first1=Sabine |last2=McGaugh |first2=Stacy S. |journal=Scientific American |volume=319 |issue=2 |pages=36–43 |doi=10.1038/scientificamerican0818-36 |pmid=30020902 |bibcode=2018SciAm.319b..36H |s2cid=51697421 |quote=Right now a few dozens of scientists are studying modified gravity, whereas several thousand are looking for particle dark matter.}}</ref> a minority of astrophysicists, intrigued by specific observations that are not well explained by ordinary dark matter, argue for various modifications of the standard laws of general relativity. These include [[modified Newtonian dynamics]], [[tensor–vector–scalar gravity]], or [[entropic gravity]]. So far none of the proposed modified gravity theories can describe [[#Observational_evidence|every piece of observational evidence]] at the same time, suggesting that even if gravity has to be modified, some form of dark matter will still be required.<ref name="CarrollTrialogue" />

== History ==

=== Early history ===
The hypothesis of dark matter has an elaborate history.<ref name=GianfracoHooperHistory/><ref>{{cite journal |last1=de Swart |first1=J.G. |last2=Bertone |first2=G. |last3=van Dongen |first3=J. |year=2017 |title=How dark matter came to matter |journal=[[Nature Astronomy]] |volume=1 |issue=59 |page=59 |arxiv=1703.00013 |bibcode=2017NatAs...1E..59D |doi=10.1038/s41550-017-0059 |s2cid=119092226 }}</ref>
[[William Thomson, 1st Baron Kelvin|Wm. Thomson, Lord Kelvin,]] discussed the potential number of stars around the Sun in the appendices of a book based on a series of lectures given in 1884 in Baltimore.<ref name=Kelvin-1904/><ref name=GianfracoHooperHistory/> He inferred their density using the observed velocity dispersion of the stars near the Sun, assuming that the Sun was 20–100&nbsp;million years old. He posed what would happen if there were a thousand million stars within 1&nbsp;[[parsec|kiloparsec]] of the Sun (at which distance their parallax would be 1&nbsp;[[minute and second of arc|milli-arcsecond]]). Kelvin concluded
<blockquote>Many of our supposed thousand million stars – perhaps a great majority of them – may be dark bodies.<ref name=Kelvin-1904>{{cite book |last=Thompson |first=((W., Lord Kelvin)) |author-link=William Thomson, 1st Baron Kelvin |year=1904 |title=Baltimore Lectures on Molecular Dynamics and the Wave Theory of Light |publisher=C.J. Clay and Sons |place=London, UK |page=274 |url=https://babel.hathitrust.org/cgi/pt?id=ien.35556038198842&view=1up&seq=304 |via=hathitrust.org }}</ref><ref name=ArsTech-2017-02-03>{{cite magazine |title=A history of dark matter  |date=2017-02-03 |df=dmy-all |magazine=[[Ars Technica]] |url=https://arstechnica.com/science/2017/02/a-history-of-dark-matter/ |access-date=8 February 2017 |lang=en-us}}</ref></blockquote>

In 1906, [[Henri Poincaré|Poincaré]]<ref name=Poincaré-1906/> used the [[French language|French]] term [''matière obscure''] ("dark matter") in discussing Kelvin's work.<ref name=Poincaré-1906>{{cite journal |last1=Poincaré |first1=H. |author-link=Henri Poincaré |year=1906 |title=La Voie lactée et la théorie des gaz |journal=Bulletin de la Société astronomique de France |volume=20 |pages=153–165 |url=https://babel.hathitrust.org/cgi/pt?id=uiug.30112110949630&view=1up&seq=171 |trans-title=The Milky Way and the theory of gases |language=fr}}</ref><ref name=ArsTech-2017-02-03/> He found that the amount of dark matter would need to be less than that of visible matter, incorrectly, it turns out.<ref name=ArsTech-2017-02-03/><ref name=GianfracoHooperHistory/>

The second to suggest the existence of dark matter using stellar velocities was Dutch astronomer [[Jacobus Kapteyn]] in 1922.<ref>{{cite journal |first=J.C. |last=Kapteyn |author-link=Jacobus Kapteyn |year=1922 |title=First attempt at a theory of the arrangement and motion of the sidereal system |journal=Astrophysical Journal |volume=55 |pages=302–327 |bibcode=1922ApJ....55..302K |doi=10.1086/142670 |quote=It is incidentally suggested when the theory is perfected it may be possible to determine ''the amount of dark matter'' from its gravitational effect. {{grey|[''emphasis in original'']}} }}</ref><ref name=Patras2014>{{cite conference |last=Rosenberg |first=Leslie J. |date=30 June 2014 |title=Status of the Axion Dark-Matter Experiment (ADMX) |conference=10th PATRAS Workshop on Axions, WIMPs and WISPs |page=2 |url=http://indico.cern.ch/event/300768/session/0/contribution/30/attachments/566901/780884/Rosenberg-Patras_30jun14.pdf |url-status=live |archive-url=https://web.archive.org/web/20160205163816/http://indico.cern.ch/event/300768/session/0/contribution/30/attachments/566901/780884/Rosenberg-Patras_30jun14.pdf |archive-date=2016-02-05 |conference-url=http://axion-wimp2014.desy.de }}</ref>

A publication from 1930 by Swedish astronomer [[Knut Lundmark]] points to him being the first to realise that the universe must contain much more mass than can be observed.<ref>{{cite journal |last=Lundmark |first=K. |author-link=Knut Lundmark |date=1930-01-01 |title=Über die Bestimmung der Entfernungen, Dimensionen, Massen, und Dichtigkeit fur die nächstgelegenen anagalacktischen Sternsysteme |lang=de |trans-title=On determination of distances, dimensions, masses, and densities for the nearest non-galactic star systems |journal=Meddelanden Fran Lunds Astronomiska Observatorium |series=I |volume=125 |pages=1–13 |bibcode=1930MeLuF.125....1L  |url=https://ui.adsabs.harvard.edu/abs/1930MeLuF.125....1L }}</ref> Dutch radio astronomy pioneer [[Jan Oort]] also hypothesized the existence of dark matter in 1932.<ref name=Patras2014/><ref>{{cite journal |last=Oort |first=J.H. |author-link=Jan Oort |year=1932 |title=The force exerted by the stellar system in the direction perpendicular to the galactic plane and some related problems |journal=Bulletin of the Astronomical Institutes of the Netherlands |volume=6 |pages=249–287 |bibcode=1932BAN.....6..249O }}</ref><ref>{{cite web |title=The hidden lives of galaxies: Hidden mass |website=Imagine the Universe |publisher=[[NASA]] / [[GSFC]] |place=Greenbelt, MD |url=http://imagine.gsfc.nasa.gov/teachers/galaxies/imagine/hidden_mass.html }}</ref> Oort was studying stellar motions in [[Local Group|the galactic neighborhood]] and found the mass in the galactic plane must be greater than what was observed, but this measurement was later determined to be erroneous.<ref>{{cite journal |last1=Kuijken |first1=K. |last2=Gilmore |first2=G. |date=July 1989 |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=239 |issue=2 |pages=651–664 |bibcode=1989MNRAS.239..651K |title=The Mass Distribution in the Galactic Disc – Part III – the Local Volume Mass Density |doi=10.1093/mnras/239.2.651 |doi-access=free }}</ref>

In 1933, Swiss astrophysicist [[Fritz Zwicky]] studied [[galaxy cluster]]s while working at [[California Institute of Technology|Cal Tech]] and made a similar inference.<ref name=zwicky1933>{{cite journal |last=Zwicky |first=F. |author-link=Fritz Zwicky |date=1933 |title=Die Rotverschiebung von extragalaktischen Nebeln |trans-title=The red shift of extragalactic nebulae |journal=[[Helvetica Physica Acta]] |volume=6 |pages=110–127 |bibcode=1933AcHPh...6..110Z }}</ref>{{efn|
''"Um, wie beobachtet, einen mittleren Dopplereffekt von 1000&nbsp;km/sek oder mehr zu erhalten, müsste also die mittlere Dichte im Comasystem mindestens 400&nbsp;mal grösser sein als die auf Grund von Beobachtungen an leuchtender Materie abgeleitete. Falls sich dies bewahrheiten sollte, würde sich also das überraschende Resultat ergeben, dass dunkle Materie in sehr viel grösserer Dichte vorhanden ist als leuchtende Materie."''<ref name=zwicky1933/>{{rp|style=ama|p= 125}}
:
: [In order to obtain an average Doppler effect of 1000&nbsp;km/s or more, as observed, the average density in the Coma system would thus have to be at least 400&nbsp;times greater than that derived on the basis of observations of luminous matter. If this were to be confirmed, the surprising result would then follow that dark matter is present in very much greater density than luminous matter.]
}}<ref name="zwicky1937">{{Cite journal |last=Zwicky |first=Fritz |author-link=Fritz Zwicky |date=1937 |title=On the Masses of Nebulae and of Clusters of Nebulae |journal=[[The Astrophysical Journal]] |volume=86 |pages=217–246 |bibcode=1937ApJ....86..217Z |doi=10.1086/143864 |doi-access=free}}</ref> Zwicky applied the [[virial theorem]] to the [[Coma Cluster]] and obtained evidence of unseen mass he called {{Lang|de|dunkle Materie}} ('dark matter'). Zwicky estimated its mass based on the motions of galaxies near its edge and compared that to an estimate based on its brightness and number of galaxies. He estimated the cluster had about 400&nbsp;times more mass than was visually observable. The gravity effect of the visible galaxies was far too small for such fast orbits, thus mass must be hidden from view. Based on these conclusions, Zwicky inferred some unseen matter provided the mass and associated gravitational attraction to hold the cluster together.<ref>Some details of Zwicky's calculation and of more modern values are given in {{cite report |first=M. |last=Richmond |date=c. 1999 |title=Using the virial theorem: The mass of a cluster of galaxies |type=lecture notes |series=Physics&nbsp;440 |publisher=[[Rochester Institute of Technology]] |place=Rochester, NY |url=http://spiff.rit.edu/classes/phys440/lectures/gal_clus/gal_clus.html |via=spiff.rit.edu |access-date=10 July 2007}}</ref> Zwicky's estimates were off by more than an order of magnitude, mainly due to an obsolete value of the [[Hubble constant]];<ref>{{cite book |first=Katherine |last=Freese |year=2014 |title=The Cosmic Cocktail: Three parts dark matter |publisher=Princeton University Press |isbn=978-1-4008-5007-5 |url={{google books |plainurl=y |id=c2B8AgAAQBAJ}} }}</ref> the same calculation today shows a smaller fraction, using greater values for luminous mass. Nonetheless, Zwicky did correctly conclude from his calculation that most of the gravitational matter present was dark.<ref name=ArsTech-2017-02-03/>

Further indications of [[mass-to-light ratio]] anomalies came from measurements of [[galaxy rotation curve]]s. In 1939, [[Horace W. Babcock|H.W. Babcock]] reported the rotation curve for the [[Andromeda Galaxy|Andromeda nebula]] (now called ''the Andromeda galaxy''), which suggested the mass-to-luminosity ratio increases radially.<ref name=Babcock-1939>{{cite journal |last=Babcock |first=H.W. |author-link=Horace W. Babcock |year=1939 |title=The rotation of the Andromeda Nebula |journal=[[Lick Observatory Bulletin]] |volume=19 |pages=41–51 |bibcode=1939LicOB..19...41B |doi=10.5479/ADS/bib/1939LicOB.19.41B |doi-access=free }}</ref> He attributed it to either light absorption within the galaxy or modified dynamics in the outer portions of the spiral, rather than to unseen matter. Following [[Horace W. Babcock|Babcock's]] 1939 report of unexpectedly rapid rotation in the outskirts of the Andromeda galaxy and a mass-to-light ratio of 50; in 1940, [[Jan Oort|Oort]] discovered and wrote about the large non-visible halo of [[NGC 3115|NGC&nbsp;3115]].<ref>{{cite journal |last=Oort |first=J.H. |author-link=Jan Oort |date=April 1940 |title=Some problems concerning the structure and dynamics of the galactic system and the elliptical nebulae NGC&nbsp;3115 and 4494 |journal=[[The Astrophysical Journal]] |volume=91 |issue=3 |pages=273–306 |bibcode=1940ApJ....91..273O |doi=10.1086/144167 |hdl=1887/8533 |hdl-access=free |url=https://openaccess.leidenuniv.nl/bitstream/handle/1887/8533/008_653_032.pdf?sequence=1 |via=leidenuniv.nl }}</ref>

=== 1960s ===
Early radio astronomy observations, performed by [[Seth Shostak]], later [[SETI Institute|SETI]] Institute Senior Astronomer, showed a half-dozen galaxies spun too fast in their outer regions, pointing to the existence of dark matter as a means of creating the gravitational pull needed to keep the stars in their orbits.<ref>{{cite web |url=https://astronomy.com/-/media/Files/PDF/Superstars/Shostak.pdf |archive-url=https://web.archive.org/web/20210927140634/https://www.astronomy.com/-/media/Files/PDF/Superstars/Shostak.pdf |archive-date=2021-09-27 |url-status=live |title=Superstars of Astronomy podcast |author=<!--Not stated--> }}</ref>

=== 1970s ===
The hypothesis of dark matter largely took root in the 1970s. Several different observations were synthesized to argue that galaxies should be surrounded by halos of unseen matter. In two papers that appeared in 1974, this conclusion was drawn in tandem by independent groups: in Princeton, New Jersey, U.S.A., by [[Jeremiah Ostriker]], [[Jim Peebles]], and Amos Yahil, and in Tartu, Estonia, by [[Jaan Einasto]], Enn Saar, and Ants Kaasik.<ref name=DeSwart2024>{{cite journal |last1=de Swart |first1=Jaco |date=1 August 2024 |title=Five decades of missing mass |journal=[[Physics Today]] |volume=77 |pages=34–43 | doi=10.1063/pt.ozhk.lfeb | url=https://doi.org/10.1063/pt.ozhk.lfeb|doi-access=free }}</ref>

One of the observations that served as evidence for the existence of galactic halos of dark matter was the shape of [[galaxy rotation curve]]s. These observations were done in optical and radio astronomy. In optical astronomy, [[Vera Rubin]] and [[Kent Ford (astronomer)|Kent Ford]] worked with a new [[spectrograph]] to measure the [[galaxy rotation curve|velocity curve]] of edge-on [[spiral galaxy|spiral galaxies]] with greater accuracy.<ref name=NYT-20161227>{{cite news |last=Overbye |first=D. |author-link=Dennis Overbye |date=27 December 2016 |title=Vera Rubin, 88, dies; opened doors in astronomy, and for women |type=obituary |newspaper=[[The New York Times]] |url=https://www.nytimes.com/2016/12/27/science/vera-rubin-astronomist-who-made-the-case-for-dark-matter-dies-at-88.html |access-date=27 December 2016 }}</ref><ref>{{cite web |title=First observational evidence of dark matter |website=Darkmatterphysics.com |url=http://www.darkmatterphysics.com/Galactic-rotation-curves-of-spiral-galaxies.htm |access-date=6 August 2013 |archive-url=https://web.archive.org/web/20130625183052/http://www.darkmatterphysics.com/Galactic-rotation-curves-of-spiral-galaxies.htm |archive-date=25 June 2013}}</ref><ref name=Rubin1970>{{cite journal |last1=Rubin |first1=V.C. |author1-link=Vera Rubin |last2=Ford |first2=W.K. Jr. |author2-link=Kent Ford (astronomer) |date=February 1970 |title=Rotation of the Andromeda nebula from a spectroscopic survey of emission regions |journal=[[The Astrophysical Journal]] |volume=159 |pages=379–403 |bibcode=1970ApJ...159..379R |doi=10.1086/150317 |s2cid=122756867 }}</ref>

At the same time, radio astronomers were making use of new [[Radio telescope|radio telescopes]] to map the 21&nbsp;cm line of atomic hydrogen in nearby galaxies. The radial distribution of interstellar atomic hydrogen ([[H I region|H{{sup|{{math|I}}}}]]) often extends to much greater galactic distances than can be observed as collective starlight, expanding the sampled distances for rotation curves – and thus of the total mass distribution – to a new dynamical regime. Early mapping of [[Andromeda galaxy|Andromeda]] with the 300&nbsp;foot telescope at [[Green Bank Observatory|Green Bank]]<ref name=Roberts1966>{{cite journal |last1=Roberts |first1=Morton S. <!-- |author-link1=Morton S. Roberts (astronomer) --> |date=May 1966 |title=A high-resolution 21&nbsp;cm hydrogen-line survey of the Andromeda nebula |journal=[[The Astrophysical Journal]] |volume=159 |pages=639–656 |bibcode=1966ApJ...144..639R |doi=10.1086/148645}}</ref> and the 250&nbsp;foot dish at [[Jodrell Bank Observatory|Jodrell Bank]]<ref name="Gottesman1966">{{cite journal |last1=Gottesman |first1=S. T. <!-- |author-link1=S. T. Gottesman (astronomer) --> |last2=Davies |first2=Rod D. |author-link2=Rod Davies |last3=Reddish |first3=Vincent Cartledge |author-link3=Vincent Cartledge Reddish |date=1966 |title=A neutral hydrogen survey of the southern regions of the Andromeda nebula |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=133 |issue=4 |pages=359–387 |bibcode=1966MNRAS.133..359G |doi=10.1093/mnras/133.4.359 |doi-access=free}}</ref> already showed the H{{sup|{{math|I}}}} rotation curve did not trace the decline expected from Keplerian orbits.

As more sensitive receivers became available, <!-- [[Morton S. Roberts|-->Roberts<!--]]--> & <!-- [[Robert N. Whitehurst (astronomer) --- not the Olympic sailor --- |-->Whitehurst<!--]]--> (1975)<ref name=Roberts1975>{{cite journal |last1=Roberts |first1=Morton S.  <!-- |author-link1=Morton S. Roberts |last2=Whitehurst |first2=Robert N.  |author-link2=Robert N. Whitehurst (astronomer) --> |date=October 1975 |title=The rotation curve and geometry of M&nbsp;31 at large galactocentric distances |journal=[[The Astrophysical Journal]] |volume=201 |pages=327–346 |bibcode=1975ApJ...201..327R |doi=10.1086/153889}}</ref> were able to trace the rotational velocity of Andromeda to 30&nbsp;kpc, much beyond the optical measurements. Illustrating the advantage of tracing the gas disk at large radii; that paper's ''Figure&nbsp;16''<ref name=Roberts1975/> combines the optical data<ref name=Rubin1970/> (the cluster of points at radii of less than 15&nbsp;kpc with a single point further out) with the H{{sup|{{math|I}}}} data between 20 and 30&nbsp;kpc, exhibiting the flatness of the outer galaxy rotation curve; the solid curve peaking at the center is the optical surface density, while the other curve shows the cumulative mass, still rising linearly at the outermost measurement. In parallel, the use of interferometric arrays for extragalactic H{{sup|{{math|I}}}} spectroscopy was being developed. <!-- [[David H. Rogstad (astronomer)| -->Rogstad<!-- ]] --> & [[Seth Shostak|Shostak]] (1972)<ref name=Rogstad1972/> published H{{sup|{{math|I}}}} rotation curves of five spirals mapped with the Owens Valley interferometer; the rotation curves of all five were very flat, suggesting very large values of mass-to-light ratio in the outer parts of their extended H{{sup|{{math|I}}}}&nbsp;disks.<ref name="Rogstad1972">{{cite journal |last1=Rogstad |first1=David H. <!-- |author-link1=David H. Rogstad (astronomer) --> |last2=Shostak |first2=G. Seth |author-link2=Seth Shostak |date=September 1972 |title=Gross properties of five Scd galaxies as determined from 21&nbsp;centimeter observations |journal=[[The Astrophysical Journal]] |volume=176 |pages=315–321 |bibcode=1972ApJ...176..315R |doi=10.1086/151636}}</ref> In 1978, Albert Bosma showed further evidence of flat rotation curves using data from the [[Westerbork Synthesis Radio Telescope]].<ref>{{cite thesis |last=Bosma |first=A. |date=1978 |title=The distribution and kinematics of neutral hydrogen in spiral galaxies of various morphological types |degree=Ph.D. |publisher=[[Rijksuniversiteit Groningen]] |url=http://nedwww.ipac.caltech.edu/level5/March05/Bosma/frames.html}}</ref> 

By the late 1970s the existence of dark matter halos around galaxies was widely recognized as real, and became a major unsolved problem in astronomy.<ref name=DeSwart2024/>

=== 1980–1990s ===
A stream of observations in the 1980–1990s supported the presence of dark matter. {{harvp|Persic|Salucci|Stel|1996}} is notable for the investigation of 967&nbsp;spirals.<ref>{{cite journal |first1=Massimo |last1=Persic |first2=Paolo |last2=Salucci |first3=Fulvio |last3=Stel |year=1996 |title=The universal rotation curve of spiral galaxies — I.&nbsp;The dark matter connection  |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=281  |issue=1 |pages=27–47 |doi= 10.1093/mnras/278.1.27 |doi-access=free |arxiv=astro-ph/9506004 |bibcode= 1996MNRAS.281...27P }}</ref> The evidence for dark matter also included [[gravitational lensing]] of background objects by [[galaxy cluster]]s,<ref name=Randall_2015/>{{rp|style=ama|pp= 14–16}} the temperature distribution of hot gas in galaxies and clusters, and the pattern of anisotropies in the [[cosmic microwave background]].

According to the current consensus among cosmologists, dark matter is composed primarily of some type of not-yet-characterized [[subatomic particle]].<ref name="Copi 1995">{{cite journal |last1=Copi |first1=C.J. |last2=Schramm |first2=D.N. |last3=Turner |first3=M.S. |year=1995 |title=Big-Bang nucleosynthesis and the baryon density of the universe |journal=[[Science (journal)|Science]] |volume=267 |issue=5195 |pages=192–199 |arxiv=astro-ph/9407006 |bibcode=1995Sci...267..192C |doi=10.1126/science.7809624 |pmid=7809624 |s2cid=15613185 |url=https://cds.cern.ch/record/265576 }}</ref><ref name="Bergstrom 2000">{{cite journal |last=Bergstrom |first=L. |year=2000 |title=Non-baryonic dark matter: Observational evidence and detection methods |journal=[[Reports on Progress in Physics]] |volume=63 |issue=5 |pages=793–841 |arxiv=hep-ph/0002126 |bibcode=2000RPPh...63..793B |doi=10.1088/0034-4885/63/5/2r3 |s2cid=119349858 }}</ref>
The search for this particle, by a variety of means, is one of the major efforts in [[particle physics]].<ref name="bertone hooper silk">{{cite journal |last1=Bertone |first1=G. |last2=Hooper |first2=D. |last3=Silk |first3=J. |year=2005 |title=Particle dark matter: Evidence, candidates, and constraints |journal=[[Physics Reports]] |volume=405 |issue=5–6 |pages=279–390 |arxiv=hep-ph/0404175 |bibcode=2005PhR...405..279B |doi=10.1016/j.physrep.2004.08.031 |s2cid=118979310 }}</ref>

== Technical definition ==
{{see also|Friedmann equations}}
In standard cosmological calculations, ''"matter"'' means any constituent of the universe whose energy density scales with the inverse cube of the [[scale factor (cosmology)|scale factor]], i.e., {{nobr|{{math|''ρ'' ∝ ''a''{{sup|−3}} }}.}} This is in contrast to ''"radiation"'', which scales as the inverse fourth power of the scale factor {{nobr|{{math|''ρ'' ∝ ''a''{{sup|−4}} }},}} and a [[cosmological constant]], which does not change with respect to {{mvar|a}} ({{nobr|{{math|''ρ'' ∝ ''a''{{sup|0}}}}}}).<ref name="Baumann lecture notes" /> The different scaling factors for matter and radiation are a consequence of radiation [[redshift]]: For example, after gradually doubling the diameter of the observable Universe via [[cosmic expansion]] of General Relativity, the scale, {{mvar|a}}, has doubled. The energy of the [[cosmic microwave background radiation]] has been halved (because the wavelength of each photon has doubled);<ref>{{cite news |last1=Siegel |first1=Ethan |title=Is energy conserved when photons redshift in our expanding universe? |year=2019 |work=[[Starts With a Bang]] |url=https://www.forbes.com/sites/startswithabang/2019/08/14/is-energy-conserved-when-photons-redshift-due-to-the-expanding-universe/?sh=745b3e3a3efa |access-date=5 November 2022 |language=en}}</ref> the energy of ultra-relativistic particles, such as early-era standard-model neutrinos, is similarly halved.{{efn|
However, in the modern cosmic era, this neutrino field has cooled and started to behave more like matter and less like radiation.
}} The cosmological constant, as an intrinsic property of space, has a constant energy density regardless of the volume under consideration.<ref name="Baumann lecture notes">{{cite web |first=Daniel |last=Baumann |title=Cosmology: Part&nbsp;III |department=Mathematical Tripos |publisher=Cambridge University |pages=21–22 |url=http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf |access-date=24 January 2017 |archive-url=https://web.archive.org/web/20170202065045/http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf |archive-date=2 February 2017 }}</ref>

In principle, "dark matter" means all components of the universe which are not visible but still obey {{nobr|{{math|''ρ'' ∝ ''a''{{sup|−3}} }}.}} In practice, the term "dark matter" is often used to mean only the non-baryonic component of dark matter, i.e., excluding "[[missing baryon problem|missing baryons]]".<ref>{{citeweb |author=Peter |first=Annika H. G. |date=18 Jan 2012 |title=Dark Matter: A Brief Review |url=https://arxiv.org/abs/1201.3942}}</ref> Context will usually indicate which meaning is intended.

== Observational evidence ==
=== Galaxy rotation curves ===
{{Main|Galaxy rotation curve}}
[[File:Comparison of rotating disc galaxies in the distant Universe and the present day.webm|thumb|Animation of rotating disc galaxies. Dark matter{{snd}}shown in red{{snd}}is more concentrated near the center and it rotates more rapidly.]]
The arms of [[spiral galaxies]] rotate around the galactic center. The luminous mass density of a spiral galaxy decreases as one goes from the center to the outskirts. If luminous mass were all the matter, then we can model the galaxy as a point mass in the centre and test masses orbiting around it, similar to the [[Solar System]].<ref group=lower-alpha>This is a consequence of the [[shell theorem]] and the observation that spiral galaxies are spherically symmetric to a large extent (in 2D).</ref> From [[Kepler's Third Law]], it is expected that the rotation velocities will decrease with distance from the center, similar to the Solar System. This is not observed.<ref>{{cite journal |last=Salucci |first=P. |title=The distribution of dark matter in galaxies |journal=[[The Astronomy and Astrophysics Review]] |date=2019 |volume= 27 |issue= 1 |pages= 2 |doi=10.1007/s00159-018-0113-1 |arxiv= 1811.08843 |bibcode= 2019A&ARv..27....2S}}</ref> Instead, the galaxy rotation curve remains flat or even increases as distance from the center increases.

If Kepler's laws are correct, then the obvious way to resolve this discrepancy is to conclude the mass distribution in spiral galaxies is not similar to that of the Solar System. In particular, there is a lot of non-luminous matter (dark matter) in the outskirts of the galaxy.

=== Velocity dispersions ===
{{Main|Velocity dispersion}}
Stars in bound systems must obey the [[virial theorem]]. The theorem, together with the measured velocity distribution, can be used to measure the mass distribution in a bound system, such as elliptical galaxies or globular clusters. With some exceptions, velocity dispersion estimates of elliptical galaxies<ref>{{cite journal |last1=Faber |first1=S. M. |last2=Jackson |first2=R. E. |date=1976 |title=Velocity dispersions and mass-to-light ratios for elliptical galaxies |journal=The Astrophysical Journal |volume=204 |pages=668–683 |bibcode=1976ApJ...204..668F |doi=10.1086/154215}}</ref> do not match the predicted velocity dispersion from the observed mass distribution, even assuming complicated distributions of stellar orbits.<ref>{{cite book |last1=Binny |first1=James |last2=Merrifield |first2=Michael |year=1998 |title=Galactic Astronomy |publisher=Princeton University Press |pages=712–713}}</ref>

As with galaxy rotation curves, the obvious way to resolve the discrepancy is to postulate the existence of non-luminous matter.

=== Galaxy clusters ===
[[Galaxy clusters]] are particularly important for dark matter studies since their masses can be estimated in three independent ways:
* From the scatter in radial velocities of the galaxies within clusters
* From [[X-ray]]s emitted by hot gas in the clusters. From the X-ray energy spectrum and flux, the gas temperature and density can be estimated, hence giving the pressure; assuming pressure and gravity balance determines the cluster's mass profile.
* [[Gravitational lens]]ing (usually of more distant galaxies) can measure cluster masses without relying on observations of dynamics (e.g., velocity).
 
Generally, these three methods are in reasonable agreement that dark matter outweighs visible matter by approximately 5 to 1.<ref>{{cite journal |title=Cosmological Parameters from Clusters of Galaxies |journal=[[Annual Review of Astronomy and Astrophysics]] |volume=49 |issue=1 |pages=409–470 |doi=10.1146/annurev-astro-081710-102514 |arxiv=1103.4829 |bibcode=2011ARA&A..49..409A |year=2011 |last1=Allen |first1=Steven W. |last2=Evrard |first2=August E. |last3=Mantz |first3=Adam B. |s2cid=54922695 }}</ref>

=== Gravitational lensing ===
One of the consequences of [[general relativity]] is the [[gravitational lens]]. Gravitational lensing occurs when massive objects between a source of light and the observer act as a lens to bend light from this source. Lensing does not depend on the properties of the mass; it only requires there to be a mass. The more massive an object, the more lensing is observed. An example is a [[galaxy cluster|cluster of galaxies]] lying between a more distant source such as a [[quasar]] and an observer. In this case, the galaxy cluster will lens the quasar.

Strong lensing is the observed distortion of background galaxies into arcs when their light passes through such a gravitational lens. It has been observed around many distant clusters including [[Abell 1689]].<ref>{{cite journal |last1=Taylor |first1=A. N. |display-authors=etal |date=1998 |title=Gravitational lens magnification and the mass of Abell&nbsp;1689 |journal=The Astrophysical Journal |volume=501 |issue=2 |pages=539–553 |arxiv=astro-ph/9801158 |bibcode=1998ApJ...501..539T |doi=10.1086/305827 |s2cid=14446661}}</ref> By measuring the distortion geometry, the mass of the intervening cluster can be obtained. In the [[weak gravitational lensing|weak]] regime, lensing does not distort background galaxies into arcs, causing minute distortions instead. By examining the apparent shear deformation of the adjacent background galaxies, the mean distribution of dark matter can be characterized. The measured mass-to-light ratios correspond to dark matter densities predicted by other large-scale structure measurements.<ref>{{cite journal |last1=Refregier |first1=A. |year=2003 |title=Weak gravitational lensing by large-scale structure |journal=[[Annual Review of Astronomy and Astrophysics]] |volume=41 |issue=1 |pages=645–668 |arxiv=astro-ph/0307212 |bibcode=2003ARA&A..41..645R |doi=10.1146/annurev.astro.41.111302.102207|s2cid=34450722 }}</ref><ref>{{Cite journal |last1=Wu |first1=X. |last2=Chiueh |first2=T. |last3=Fang |first3=L. |last4=Xue |first4=Y. |date=1998 |title=A comparison of different cluster mass estimates: consistency or discrepancy? |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=301 |issue=3 |pages=861–871 |arxiv=astro-ph/9808179 |bibcode=1998MNRAS.301..861W |doi=10.1046/j.1365-8711.1998.02055.x |doi-access=free |citeseerx=10.1.1.256.8523|s2cid=1291475 }}</ref>

=== Cosmic microwave background ===
{{Main|Cosmic microwave background}}
Although both dark matter and ordinary matter are matter, they do not behave in the same way. In particular, in the early universe, ordinary matter was ionized and interacted strongly with radiation via [[Thomson scattering]]. Dark matter does not interact directly with radiation, but it does affect the cosmic microwave background (CMB) by its gravitational potential (mainly on large scales) and by its effects on the density and velocity of ordinary matter. Ordinary and dark matter perturbations, therefore, evolve differently with time and leave different imprints on the CMB.

The CMB is very close to a perfect blackbody but contains very small temperature anisotropies of a few parts in 100,000. A sky map of anisotropies can be decomposed into an angular power spectrum, which is observed to contain a series of acoustic peaks at near-equal spacing but different heights. The locations of these peaks depend on cosmological parameters. Matching theory to data, therefore, constrains cosmological parameters.<ref name="Wayne Hu">The details are technical. For an intermediate-level introduction, see {{cite web |author=Hu, Wayne |title=Intermediate Guide to the Acoustic Peaks and Polarization |url=http://background.uchicago.edu/~whu/intermediate/intermediate.html |year=2001}}</ref>

The CMB anisotropy was first discovered by [[Cosmic Background Explorer|COBE]] in 1992, though this had too coarse resolution to detect the acoustic peaks.
After the discovery of the first acoustic peak by the balloon-borne [[BOOMERanG]] experiment in 2000, the power spectrum was precisely observed by [[WMAP]] in 2003–2012, and even more precisely by the [[Planck spacecraft|''Planck'' spacecraft]] in 2013–2015. The results support the Lambda-CDM model.<ref name="Hinshaw2009">{{Cite journal |last1=Hinshaw |first1=G. |display-authors=etal |year=2009 |title=Five-year Wilkinson microwave anisotropy probe (WMAP) observations: Data processing, sky maps, and basic results |journal=The Astrophysical Journal Supplement |volume=180 |issue=2 |pages=225–245 |arxiv=0803.0732 |bibcode=2009ApJS..180..225H |doi=10.1088/0067-0049/180/2/225|s2cid=3629998 }}</ref><ref name="Planck15" />

The observed CMB angular power spectrum provides powerful evidence in support of dark matter, as its precise structure is well fitted by the [[lambda-CDM model]],<ref name="Planck15"/> but difficult to reproduce with any competing model such as [[modified Newtonian dynamics]] (MOND).<ref name="Planck15">{{cite journal |last1=Ade |first1=P.A.R. |display-authors=etal |title=Planck 2015 results. XIII. Cosmological parameters |journal=Astron. Astrophys. |year=2016 |volume=594 |issue=13 |page=A13 |doi=10.1051/0004-6361/201525830 |bibcode=2016A&A...594A..13P |arxiv=1502.01589 |s2cid=119262962 }}</ref><ref>{{cite journal |last1=Skordis |first1=C. |display-authors=etal |title=Large scale structure in Bekenstein's theory of relativistic modified Newtonian dynamics |journal=Phys. Rev. Lett. |year=2006 |volume=96 |issue=1 |page=011301 |doi=10.1103/PhysRevLett.96.011301 |pmid=16486433 |bibcode=2006PhRvL..96a1301S |arxiv=astro-ph/0505519|s2cid=46508316 }}</ref>

=== Structure formation ===
{{Main|Structure formation}}
[[File:Dark matter map of KiDS survey region (region G12).jpg|right|thumb|Dark matter map for a patch of sky based on gravitational lensing analysis of a Kilo-Degree survey<ref>{{cite web |title=Dark matter may be smoother than expected – Careful study of large area of sky imaged by VST reveals intriguing result |url=https://www.eso.org/public/news/eso1642/ |access-date=8 December 2016 |website=www.eso.org}}</ref>]]Structure formation refers to the period after the [[Big Bang]] when density perturbations collapsed to form stars, galaxies, and clusters. Prior to structure formation, the [[FRW metric|Friedmann solutions]] to general relativity describe a homogeneous universe. Later, small anisotropies gradually grew and condensed the homogeneous universe into stars, galaxies and larger structures. Ordinary matter is affected by radiation, which is the dominant element of the universe at very early times. As a result, its density perturbations are washed out and unable to condense into structure.<ref name="Jaffe">{{cite web |author=Jaffe |first=A. H. |title=Cosmology 2012: Lecture Notes |url=http://astro.imperial.ac.uk/sites/default/files/cosmology.pdf |archive-url=https://web.archive.org/web/20160717223916/http://astro.imperial.ac.uk/sites/default/files/cosmology.pdf |archive-date=July 17, 2016}}</ref> If there were only ordinary matter in the universe, there would not have been enough time for density perturbations to grow into the galaxies and clusters currently seen.

Dark matter provides a solution to this problem because it is unaffected by radiation. Therefore, its density perturbations can grow first. The resulting gravitational potential acts as an attractive [[potential well]] for ordinary matter collapsing later, speeding up the structure formation process.<ref name="Jaffe" /><ref>{{cite journal |author=Low |first=L. F. |date=12 October 2016 |title=Constraints on the composite photon theory |url=https://zenodo.org/record/896052 |journal=Modern Physics Letters A |volume=31 |issue=36 |page=1675002 |bibcode=2016MPLA...3175002L |doi=10.1142/S021773231675002X}}</ref>

=== Bullet Cluster ===
{{Main|Bullet Cluster}}
The Bullet Cluster is the result of a recent collision of two galaxy clusters. It is of particular note because the location of the [[center of mass]] as measured by gravitational lensing is different from the location of the center of mass of visible matter. This is difficult for modified gravity theories, which generally predict lensing around visible matter, to explain.<ref>{{cite conference |url=http://cosis.net/abstracts/COSPAR2006/02655/COSPAR2006-A-02655.pdf |archive-url=https://web.archive.org/web/20060821074820/http://www.cosis.net/abstracts/COSPAR2006/02655/COSPAR2006-A-02655.pdf |archive-date=2006-08-21 |url-status=live |title=Dark matter and the Bullet Cluster |author1=Markevitch, M. |author2=Randall, S. |author3=Clowe, D. |author4=Gonzalez, A. |author5=Bradac, M. |name-list-style=amp |conference=36th COSPAR Scientific Assembly |date=16–23 July 2006 |location=Beijing, China}} Abstract only</ref><ref>{{cite journal |last1=Clowe |first1=Douglas |display-authors=etal |year=2006 |title=A Direct Empirical Proof of the Existence of Dark Matter |journal=The Astrophysical Journal Letters |volume=648 |issue=2 |pages=L109–L113 |arxiv=astro-ph/0608407 |doi=10.1086/508162 |bibcode=2006ApJ...648L.109C|s2cid=2897407 }}</ref><ref>{{cite web |url=https://arstechnica.com/science/2017/09/science-in-progress-did-the-bullet-cluster-withstand-scrutiny/ |title=Science-in-progress: Did the Bullet Cluster withstand scrutiny? |website=Ars Technica |author=Lee, Chris |date=21 September 2017}}</ref><ref>{{cite magazine |url=https://www.forbes.com/sites/startswithabang/2017/11/09/the-bullet-cluster-proves-dark-matter-exists-but-not-for-the-reason-most-physicists-think/#3032b6081738 |title=The Bullet Cluster proves dark matter exists, but not for the reason most physicists think |magazine=Forbes |author=Siegel, Ethan |author-link=Ethan Siegel |date=9 November 2017}}</ref> Standard dark matter theory however has no issue: the hot, visible gas in each cluster would be cooled and slowed down by electromagnetic interactions, while dark matter (which does not interact electromagnetically) would not. This leads to the dark matter separating from the visible gas, producing the separate lensing peak as observed.<ref>{{citeweb|url=https://chandra.harvard.edu/graphics/resources/handouts/lithos/bullet_lithos.pdf|title=Bullet Cluster: Direct Proof of Dark Matter|publisher=NASA}}</ref>

=== Type Ia supernova distance measurements ===
{{Main|Type Ia supernova|Shape of the universe}}
Type Ia [[supernovae]] can be used as [[standard candles]] to measure extragalactic distances, which can in turn be used to measure how fast the universe has expanded in the past.<ref>{{cite journal |last1=Planck Collaboration |last2=Aghanim |first2=N.|author2-link=Nabila Aghanim |last3=Akrami |first3=Y. |last4=Ashdown |first4=M. |last5=Aumont |first5=J. |last6=Baccigalupi |first6=C. |last7=Ballardini |first7=M. |last8=Banday |first8=A. J. |last9=Barreiro |first9=R. B. |last10=Bartolo |first10=N. |last11=Basak |first11=S. |title=Planck 2018 results. VI. Cosmological parameters |journal=Astronomy & Astrophysics |year=2020 |volume=641 |pages=A6 |doi=10.1051/0004-6361/201833910 |arxiv=1807.06209 |bibcode=2020A&A...641A...6P |s2cid=119335614}}</ref> Data indicates<!--data is "usually used with a singular verb". See https://www.dictionary.com/browse/data--> the universe is expanding at an accelerating rate, the cause of which is usually ascribed to [[dark energy]].<ref>{{Cite journal |last1=Kowalski |first1=M. |display-authors=etal |year=2008 |title=Improved Cosmological Constraints from New, Old, and Combined Supernova Data Sets |journal=The Astrophysical Journal |volume=686 |issue=2 |pages=749–778 |arxiv=0804.4142 |bibcode=2008ApJ...686..749K |doi=10.1086/589937 |s2cid=119197696 }}</ref> Since observations indicate the universe is almost flat,<ref name="NASA_Shape">{{cite web|url= https://map.gsfc.nasa.gov/universe/uni_shape.html |title=Will the Universe expand forever? |publisher=NASA |date=24 January 2014 |access-date=2021-03-28}}</ref><ref name="Fermi_Flat">{{cite web|url= https://www.symmetrymagazine.org/article/april-2015/our-flat-universe |title=Our flat universe |publisher=FermiLab/SLAC |date=7 April 2015 |access-date=2021-03-28}}</ref><ref>{{cite journal |title=Unexpected connections |first=Marcus Y. |last=Yoo |journal=Engineering & Science |volume=74 |issue=1 |date=2011 |page=30}}</ref> it is expected the total energy density of everything in the universe should sum to 1 ({{nowrap|Ω<sub>tot</sub> ≈ 1}}). The measured dark energy density is {{nowrap|Ω<sub>Λ</sub> ≈ 0.690}}; the observed ordinary (baryonic) matter energy density is {{nowrap|Ω<sub>b</sub> ≈ 0.0482}} and the energy density of radiation is negligible. This leaves a missing {{nowrap|Ω<sub>dm</sub> ≈ 0.258}} which nonetheless behaves like matter (see technical definition section above){{snd}}dark matter.<ref name="planckesa2015">{{cite web |url=http://www.cosmos.esa.int/web/planck/publications |title=Planck Publications: Planck 2015 Results |publisher=European Space Agency |date=February 2015 |access-date=9 February 2015}}</ref>

=== Sky surveys and baryon acoustic oscillations ===
{{Main|Baryon acoustic oscillations}}
Baryon acoustic oscillations (BAO) are fluctuations in the density of the visible baryonic matter (normal matter) of the universe on large scales. These are predicted to arise in the Lambda-CDM model due to acoustic oscillations in the photon–baryon fluid of the early universe and can be observed in the cosmic microwave background angular power spectrum. BAOs set up a preferred length scale for baryons. As the dark matter and baryons clumped together after recombination, the effect is much weaker in the galaxy distribution in the nearby universe, but is detectable as a subtle (≈1&nbsp;percent) preference for pairs of galaxies to be separated by 147&nbsp;Mpc, compared to those separated by 130–160&nbsp;Mpc. This feature was predicted theoretically in the 1990s and then discovered in 2005, in two large galaxy redshift surveys, the [[Sloan Digital Sky Survey]] and the [[2dF Galaxy Redshift Survey]].<ref>
{{Cite journal |last1=Percival |first1=W. J. |display-authors=etal |year=2007 |title=Measuring the Baryon Acoustic Oscillation scale using the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=381 |issue=3 |pages=1053–1066 |arxiv=0705.3323 |bibcode=2007MNRAS.381.1053P |doi=10.1111/j.1365-2966.2007.12268.x |doi-access=free}}</ref> Combining the CMB observations with BAO measurements from galaxy [[redshift survey]]s provides a precise estimate of the [[Hubble's law|Hubble constant]] and the average matter density in the Universe.<ref name="Komatsu2009">{{Cite journal |last1=Komatsu |first1=E. |display-authors=etal |year=2009 |title=Five-Year Wilkinson Microwave Anisotropy Probe Observations: Cosmological Interpretation |journal=The Astrophysical Journal Supplement|volume=180 |issue=2 |pages=330–376 |arxiv=0803.0547 |bibcode=2009ApJS..180..330K |doi=10.1088/0067-0049/180/2/330|s2cid=119290314 }}</ref> The results support the Lambda-CDM model.

=== Redshift-space distortions ===
Large galaxy [[redshift survey]]s may be used to make a three-dimensional map of the galaxy distribution. These maps are slightly distorted because distances are estimated from observed [[redshift]]s; the redshift contains a contribution from the galaxy's so-called peculiar velocity in addition to the dominant Hubble expansion term. On average, superclusters are expanding more slowly than the cosmic mean due to their gravity, while voids are expanding faster than average. In a redshift map, galaxies in front of a supercluster have excess radial velocities towards it and have redshifts slightly higher than their distance would imply, while galaxies behind the supercluster have redshifts slightly low for their distance. This effect causes superclusters to appear squashed in the radial direction, and likewise voids are stretched. Their angular positions are unaffected. This effect is not detectable for any one structure since the true shape is not known, but can be measured by averaging over many structures. It was predicted quantitatively by Nick Kaiser in 1987, and first decisively measured in 2001 by the [[2dF Galaxy Redshift Survey]].<ref>{{cite journal |last1=Peacock |first1=J. |display-authors=etal |title=A measurement of the cosmological mass density from clustering in the 2dF Galaxy Redshift Survey |journal=Nature |year=2001 |volume=410 |issue=6825 |pages=169–173 |arxiv=astro-ph/0103143 |bibcode=2001Natur.410..169P |doi=10.1038/35065528 |pmid=11242069|s2cid=1546652 }}</ref> Results are in agreement with the [[lambda-CDM model]].

=== Lyman-alpha forest ===
{{Main|Lyman-alpha forest}}
In [[astronomical spectroscopy]], the Lyman-alpha forest is the sum of the [[spectral line|absorption lines]] arising from the [[Lyman series|Lyman-alpha]] transition of [[hydrogen line|neutral hydrogen]] in the spectra of distant [[galaxy|galaxies]] and [[quasar]]s. Lyman-alpha forest observations can also constrain cosmological models.<ref>{{Cite journal |last1=Viel |first1=M. |last2=Bolton |first2=J. S. |last3=Haehnelt |first3=M. G. |date=2009 |title=Cosmological and astrophysical constraints from the Lyman α forest flux probability distribution function |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=399 |issue=1 |pages=L39–L43 |arxiv=0907.2927 |bibcode=2009MNRAS.399L..39V |doi=10.1111/j.1745-3933.2009.00720.x |s2cid=12470622 |doi-access=free}}</ref> These constraints agree with those obtained from WMAP data.

[[File:Dark matter candidates.pdf|thumb|upright=1.9|Different dark matter candidates as a function of their mass in units of electronvolt (eV)]]
== Theoretical classifications ==

=== Composition ===
The identity of dark matter is unknown, but there are many [[hypothesis|hypotheses]] about what dark matter could consist of, as set out in the table below.
{{clear}}
{| class = wikitable
|+ Some dark matter hypotheses
|-
|rowspan=3| [[Light boson]]s
| [[quantum chromodynamics]] [[axion]]s
|-
| [[WISP (particle physics)|axion-like particle]]s
|-
| [[fuzzy cold dark matter]]
|-
|rowspan=2| [[neutrino]]s
| [[neutrino|Standard Model]]{{efn|The three neutrino types already observed are indeed abundant, and dark, and matter, but their individual masses are almost certainly too tiny to account for more than a small fraction of dark matter, due to limits derived from [[observable universe|large-scale structure]] and high-[[redshift]] galaxies.<ref name="bertone merritt" />}}
|-
| [[sterile neutrinos]]
|-
|rowspan=5| [[weak scale]]
| [[supersymmetry]]
|-
| [[extra dimensions]]
|-
| [[little Higgs]]
|-
| [[effective field theory]]
|-
| simplified models
|-
|rowspan=3| other particles
| [[weakly interacting massive particle]]
|-
| [[self-interacting dark matter]]
|-
| [[atomic dark matter]]<ref>{{Cite journal |last1=Bansal |first1=Saurabh |last2=Barron |first2=Jared |last3=Curtin |first3=David |last4=Tsai |first4=Yuhsin |date=2023-10-16 |title=Precision cosmological constraints on atomic dark matter |url=https://doi.org/10.1007/JHEP10(2023)095 |journal=Journal of High Energy Physics |language=en |volume=2023 |issue=10 |pages=95 |doi=10.1007/JHEP10(2023)095 |arxiv=2212.02487 |bibcode=2023JHEP...10..095B |issn=1029-8479}}</ref><ref>{{Citation |last1=Bansal |first1=Saurabh |title=Precision Cosmological Constraints on Atomic Dark Matter |date=2023-07-27 |arxiv=2212.02487 |last2=Barron |first2=Jared |last3=Curtin |first3=David |last4=Tsai |first4=Yuhsin |journal=Journal of High Energy Physics |volume=2023 |issue=10 |page=95 |doi=10.1007/JHEP10(2023)095 |bibcode=2023JHEP...10..095B |quote=leading to a better fit than ΛCDM or ΛCDM + dark radiation}}</ref><ref>{{Cite web |last=Sutter |first=Paul Sutter |date=2023-06-07 |title=Dark matter atoms may form shadowy galaxies with rapid star formation |url=https://www.space.com/dark-matter-atoms-form-stars-galaxies-simulations |access-date=2024-01-09 |website=Space.com |language=en}}</ref><ref name="MirrorStars">{{cite journal |last1=Armstrong |first=Isabella |display-authors=etal |date=2024 |title=Electromagnetic Signatures of Mirror Stars |journal=The Astrophysical Journal |volume=965 |issue=1 |page=42 |arxiv=2311.18086 |bibcode=2024ApJ...965...42A |doi=10.3847/1538-4357/ad283c |doi-access=free}}</ref>
|-
| [[strangelet]]<ref>{{Cite journal |last1=VanDevender |first1=J. Pace |last2=VanDevender |first2=Aaron P. |last3=Sloan |first3=T. |last4=Swaim |first4=Criss |last5=Wilson |first5=Peter |last6=Schmitt |first6=Robert G. |last7=Zakirov |first7=Rinat |last8=Blum |first8=Josh |last9=Cross |first9=James L. |last10=McGinley |first10=Niall |date=2017-08-18 |title=Detection of magnetized quark-nuggets, a candidate for dark matter |journal=Scientific Reports |language=en |volume=7 |issue=1 |page=8758 |doi=10.1038/s41598-017-09087-3 |pmid=28821866 |pmc=5562705 |arxiv=1708.07490 |bibcode=2017NatSR...7.8758V |issn=2045-2322}}</ref>
|-
| [[superfluid vacuum theory]]
|
|-
| [[dynamical dark matter]]<ref name=DienesThomas2012>{{Cite journal |last=Dienes |first=Keith R. |last2=Thomas |first2=Brooks |date=2012-04-24 |title=Dynamical dark matter. I. Theoretical overview |url=https://journals.aps.org/prd/abstract/10.1103/PhysRevD.85.083523 |journal=Physical Review D |volume=85 |issue=8 |pages=083523 |doi=10.1103/PhysRevD.85.083523|arxiv=1106.4546 }}</ref>
|
|-
|rowspan=3| [[Macroscopic scale|macroscopic]]
| [[primordial black hole]]s<ref name="Carr24"/><ref name="Bird"/><ref name="jwst">{{cite journal |last1=Hütsi |first1=Gert |last2=Raidal |first2=Martti |last3=Urrutia |first3=Juan |last4=Vaskonen |first4=Ville |last5=Veermäe |first5=Hardi |date=2 February 2023 |title=Did JWST observe imprints of axion miniclusters or primordial black holes? |journal=Physical Review D |volume=107 |issue=4 |page=043502 |arxiv=2211.02651 |bibcode=2023PhRvD.107d3502H |doi=10.1103/PhysRevD.107.043502 |s2cid=253370365}}</ref><ref name="Carr">{{cite journal |last1=Carr |first1=Bernard |last2=Kühnel |first2=Florian |title=Primordial black holes as dark matter candidates |journal=SciPost Physics Lecture Notes |date=2 May 2022 |page=48 |doi=10.21468/SciPostPhysLectNotes.48 |s2cid=238407875 |url=https://scipost.org/SciPostPhysLectNotes.48/pdf |access-date=13 February 2023 |doi-access=free |arxiv=2110.02821 }} (See also the [https://indico.cern.ch/event/949654/contributions/4031007/attachments/2293539/3901659/Carr-Kuhnel.pdf accompanying slide presentation.]</ref><ref name="Espinosa">{{cite journal |last1=Espinosa |first1=J. R. |last2=Racco |first2=D. |last3=Riotto |first3=A. |title=A Cosmological Signature of the Standard Model Higgs Vacuum Instability: Primordial Black Holes as Dark Matter |journal=Physical Review Letters |volume=120 |issue=12 |page=121301 |doi=10.1103/PhysRevLett.120.121301 |pmid=29694085 |date=23 March 2018|arxiv=1710.11196 |bibcode=2018PhRvL.120l1301E |s2cid=206309027 }}</ref><ref name="Clesse">{{cite journal |last1=Clesse |first1=Sebastien |last2=García-Bellido |first2=Juan |title=Seven Hints for Primordial Black Hole Dark Matter |journal=Physics of the Dark Universe |volume=22 |pages=137–146 |arxiv=1711.10458 |bibcode=2018PDU....22..137C |doi=10.1016/j.dark.2018.08.004 |year=2018 |s2cid=54594536 }}</ref><ref name="Lacki">{{cite journal |last1=Lacki |first1=Brian C. |last2=Beacom |first2=John F. |title=Primordial Black Holes as Dark Matter: Almost All or Almost Nothing |journal=The Astrophysical Journal |date=12 August 2010 |volume=720 |issue=1 |pages=L67–L71 |doi=10.1088/2041-8205/720/1/L67 |language=en |issn=2041-8205 |arxiv=1003.3466 |bibcode=2010ApJ...720L..67L |s2cid=118418220 }}</ref><ref name="Kashlinsky">{{cite journal |last1=Kashlinsky |first1=A. |title=LIGO gravitational wave detection, primordial black holes and the near-IR cosmic infrared background anisotropies |journal=The Astrophysical Journal |date=23 May 2016 |volume=823 |issue=2 |pages=L25 |doi=10.3847/2041-8205/823/2/L25 |issn=2041-8213|arxiv=1605.04023 |bibcode=2016ApJ...823L..25K |s2cid=118491150 |doi-access=free }}</ref><ref name="Frampton">{{cite journal |last1=Frampton |first1=Paul H. |last2=Kawasaki |first2=Masahiro |last3=Takahashi |first3=Fuminobu |last4=Yanagida |first4=Tsutomu T. |title=Primordial Black Holes as All Dark Matter |journal=Journal of Cosmology and Astroparticle Physics |date=22 April 2010 |volume=2010 |issue=4 |page=023 |doi=10.1088/1475-7516/2010/04/023 |issn=1475-7516|arxiv=1001.2308 |bibcode=2010JCAP...04..023F |s2cid=119256778 }}</ref><ref name="Carneiro">{{cite journal |last1=Carneiro |first1=S. |last2=de Holanda |first2=P.C. |last3=Saa |first3=A. |title=Neutrino primordial Planckian black holes |journal=Physics Letters |date=2021 |volume=B822 |page=136670 |doi=10.1016/j.physletb.2021.136670 |issn=0370-2693|bibcode=2021PhLB..82236670C |s2cid=244196281 |doi-access=free |hdl=20.500.12733/1987 |hdl-access=free }}</ref>
|-
| [[massive compact halo objects]] (MACHOs)
|-
| [[macroscopic dark matter]] (Macros)
|-
|rowspan=3| [[modified gravity]] (MOG)
| [[modified Newtonian dynamics]] (MoND)
|-
| [[tensor–vector–scalar gravity]] (TeVeS)
|-
| [[entropic gravity]]
|}

[[File:Fermi Observations of Dwarf Galaxies Provide New Insights on Dark Matter.ogv|thumb|[[Fermi Gamma-ray Space Telescope|Fermi-LAT]] observations of dwarf galaxies provide new insights on dark matter.]]

==== Baryonic matter ====
{{Distinguish|Missing baryon problem}}

Dark matter can refer to any substance which interacts predominantly via gravity with visible matter (e.g., stars and planets). Hence in principle it need not be composed of a new type of fundamental particle but could, at least in part, be made up of standard [[baryon|baryonic matter]], such as protons or neutrons. Most of the ordinary matter familiar to astronomers, including planets, brown dwarfs, red dwarfs, visible stars, white dwarfs, neutron stars, and black holes, fall into this category.<ref name=GianfracoHooperHistory>{{cite journal |last1=Bertone |first1=Gianfranco |last2=Hooper |first2=Dan |title=History of dark matter |journal=Reviews of Modern Physics |date=15 October 2018 |volume=90 |issue=4 |page=045002 |doi=10.1103/RevModPhys.90.045002|arxiv=1605.04909 |bibcode=2018RvMP...90d5002B |s2cid=18596513 }}</ref><ref name=BaryonicSource01>{{cite web |url=http://astronomy.swin.edu.au/cosmos/B/Baryonic+Matter |title=Baryonic Matter |website=COSMOS – The SAO Encyclopedia of Astronomy |publisher=[[Swinburne University of Technology]]|access-date=16 November 2022}}</ref> A black hole would ingest both baryonic and non-baryonic matter that comes close enough to its event horizon; afterwards, the distinction between the two is lost.<ref>{{Cite web |title=Baryonic Matter |url=https://astronomy.swin.edu.au/cosmos/b/Baryonic+Matter |access-date=2023-10-03 |website=astronomy.swin.edu.au |publisher=Cosmos: The Swinburne Astronomy Online Encyclopedia |publication-place=Melbourne, Victoria, Australia: Swinburne University of Technology}}</ref> 

These massive objects that are hard to detect are collectively known as [[Massive compact halo object|MACHOs]]. Some scientists initially hoped that baryonic MACHOs could account for and explain all the dark matter.<ref name=Randall_2015/>{{rp|286}}<ref>{{cite news |title=MACHOs may be out of the running as a dark matter candidate |url=https://astronomy.com/news/2016/08/machos-may-be-out-of-the-running-as-a-dark-matter-candidate |access-date=16 November 2022 |work=Astronomy.com |date=2016 |language=en}}</ref>

However, multiple lines of evidence suggest the majority of dark matter is not baryonic:
* Sufficient diffuse, baryonic gas or dust would be visible when backlit by stars.
* The theory of [[Big Bang nucleosynthesis]] predicts the observed [[abundance of the chemical elements]]. If there are more baryons, then there should also be more helium, lithium and heavier elements synthesized during the Big Bang.<ref>{{cite book |author=Weiss, Achim |url=http://www.einstein-online.info/spotlights/BBN |title=Big bang nucleosynthesis: Cooking up the first light elements |archive-url=https://web.archive.org/web/20130206021217/http://www.einstein-online.info/spotlights/BBN |archive-date=6 February 2013 |publisher=Einstein Online |volume=2 |year=2006 |page=1017 |access-date=1 June 2013 |url-status=dead }}</ref><ref>{{cite book |last1=Raine |first1=D. |last2=Thomas |first2=T. |date=2001 |title=An Introduction to the Science of Cosmology |page=30 |publisher=[[IOP Publishing]] |isbn=978-0-7503-0405-4 |oclc=864166846 }}</ref> Agreement with observed abundances requires that baryonic matter makes up between 4–5% of the universe's [[Friedmann equations#Density parameter|critical density]]. In contrast, [[large-scale structure of the universe|large-scale structure]] and other observations indicate that the total matter density is about 30% of the critical density.<ref name="planckesa2015" />
* Astronomical searches for [[gravitational microlensing]] in the [[Milky Way]] found at most only a small fraction of the dark matter may be in dark, compact, conventional objects (MACHOs, etc.); the excluded range of object masses is from half the Earth's mass up to 30&nbsp;solar masses, which covers nearly all the plausible candidates.<ref>{{cite journal |last1=Tisserand |first1=P. |last2=Le&nbsp;Guillou |first2=L. |last3=Afonso |first3=C. |last4=Albert |first4=J.N. |last5=Andersen |first5=J. |last6=Ansari |first6=R. |last7=Aubourg |first7=É. |last8=Bareyre |first8=P. |last9=Beaulieu |first9=J.P. |last10=Charlot |first10=X. |last11=Coutures |first11=C. |last12=Ferlet |first12=R. |last13=Fouqué |first13=P. |last14=Glicenstein |first14=J.F. |last15=Goldman |first15=B. |last16=Gould |first16=A. |last17=Graff |first17=D. |last18=Gros |first18=M. |last19=Haissinski |first19=J. |last20=Hamadache |first20=C. |last21=De Kat |first21=J. |last22=Lasserre |first22=T. |last23=Lesquoy |first23=É. |last24=Loup |first24=C. |last25=Magneville |first25=C. |last26=Marquette |first26=J.B. |last27=Maurice |first27=É. |last28=Maury |first28=A. |last29=Milsztajn |first29=A. |last30=Moniez |first30=M. |display-authors=6 |title=Limits on the Macho content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds |journal=Astronomy and Astrophysics |volume=469 |issue=2 |pages=387–404 |year=2007 |doi=10.1051/0004-6361:20066017 |url=https://www.researchgate.net/publication/41714676 |arxiv=astro-ph/0607207 |bibcode=2007A&A...469..387T|s2cid=15389106 }}</ref><ref>{{Cite journal |last1=Graff |first1=D. S. |last2=Freese |first2=K. |year=1996 |title=Analysis of a ''Hubble Space Telescope'' Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo |journal=The Astrophysical Journal |volume=456 |issue=1996 |page=L49 |arxiv=astro-ph/9507097 |bibcode=1996ApJ...456L..49G |doi=10.1086/309850 |s2cid=119417172}}</ref><ref>{{cite journal |last1=Najita |first1=J. R. |last2=Tiede |first2=G. P. |last3=Carr |first3=J. S. |year=2000 |title=From Stars to Superplanets: The Low-Mass Initial Mass Function in the Young Cluster IC 348 |journal=The Astrophysical Journal |volume=541 |issue=2 |pages=977–1003 |arxiv=astro-ph/0005290 |bibcode=2000ApJ...541..977N |doi=10.1086/309477 |s2cid=55757804}}</ref><ref>{{cite journal |arxiv=1106.2925 |title=The OGLE View of Microlensing towards the Magellanic Clouds. IV. OGLE-III SMC Data and Final Conclusions on MACHOs |journal=Monthly Notices of the Royal Astronomical Society |volume=416 |issue=4 |pages=2949–2961 |last1=Wyrzykowski |first1=L. |last2=Skowron |first2=J. |last3=Kozlowski |first3=S. |last4=Udalski |first4=A. |last5=Szymanski |first5=M.K. |last6=Kubiak |first6=M. |last7=Pietrzynski |first7=G. |last8=Soszynski |first8=I. |last9=Szewczyk |first9=O. |last10=Ulaczyk |first10=K. |last11=Poleski |first11=R. |last12=Tisserand |first12=P. |display-authors=6 |doi=10.1111/j.1365-2966.2011.19243.x |year=2011 |doi-access=free |bibcode=2011MNRAS.416.2949W|s2cid=118660865 }}</ref><ref>{{cite arXiv |title=Death of stellar baryonic dark matter candidates |first1=Katherine |last1=Freese |eprint=astro-ph/0007444 |last2=Fields |first2=Brian |last3=Graff |first3=David |year=2000}}</ref><ref>{{cite book |pages=4–6 |first1=Katherine |last1=Freese |arxiv=astro-ph/0002058 |bibcode=2000fist.conf...18F |doi=10.1007/10719504_3 |chapter=Death of Stellar Baryonic Dark Matter |series=ESO Astrophysics Symposia |last2=Fields |first2=Brian |last3=Graff |first3=David |year=2003 |title=The First Stars |isbn=978-3-540-67222-7 |citeseerx=10.1.1.256.6883|s2cid=119326375 }}</ref>
* Detailed analysis of the small irregularities (anisotropies) in the [[cosmic microwave background]] by [[WMAP]] and [[Planck (spacecraft)|Planck]] indicate that around five-sixths of the total matter is in a form that only interacts significantly with ordinary matter or [[photon]]s through gravitational effects.<ref>{{cite journal |first1=L. |last1=Canetti |first2=M. |last2=Drewes |first3=M. |last3=Shaposhnikov |title=Matter and Antimatter in the Universe |journal=New J. Phys. |year=2012 |volume=14 |issue=9 |page=095012 |doi=10.1088/1367-2630/14/9/095012 |arxiv=1204.4186 |bibcode=2012NJPh...14i5012C|s2cid=119233888 }}</ref>

==== Non-baryonic matter ====
There are two main candidates for non-baryonic dark matter: new hypothetical particles and [[primordial black hole]]s. 

Unlike baryonic matter, nonbaryonic particles do not contribute to the formation of the [[Chemical element|elements]] in the early universe ([[Big Bang nucleosynthesis]])<ref name="Copi 1995" /> and so its presence is revealed only via its gravitational effects, or [[weak lensing]]. In addition, if the particles of which it is composed are supersymmetric, they can undergo [[annihilation]] interactions with themselves, possibly resulting in observable by-products such as [[gamma rays]] and neutrinos (indirect detection).<ref name="bertone merritt">{{Cite journal |last1=Bertone |first1=G. |last2=Merritt |first2=D. |title=Dark Matter Dynamics and Indirect Detection |year=2005 |journal=[[Modern Physics Letters A]] |volume=20 |issue=14 |pages=1021–1036 |arxiv=astro-ph/0504422 |bibcode=2005MPLA...20.1021B |doi=10.1142/S0217732305017391|s2cid=119405319 }}</ref>

In 2015, the idea that dense dark matter was composed of primordial black holes made a comeback<ref>
{{cite journal
 |last1=Cho |first1=Adrian
 |date=9 February 2017
 |title=Is dark matter made of black holes?
 |journal=[[Science (journal)|Science]]
 |doi=10.1126/science.aal0721
}}
</ref> following results of [[gravitational wave]] measurements which detected the merger of intermediate-mass black holes. Black holes with about 30&nbsp;solar masses are not predicted to form by either stellar collapse (typically less than 15&nbsp;solar masses) or by the merger of black holes in galactic centers (millions or billions of solar masses). It was proposed that the intermediate-mass black holes causing the detected merger formed in the hot dense early phase of the universe due to denser regions collapsing. A later survey of about a thousand supernovae detected no gravitational lensing events, when about eight would be expected if intermediate-mass primordial black holes above a certain mass range accounted for over 60% of dark matter.<ref>
{{cite news
 |title=Black holes can't explain dark matter
 |date=18 October 2018
 |magazine=[[Astronomy (magazine)|Astronomy]]
 |via=astronomy.com
 |url=http://astronomy.com/news/2018/10/can-black-holes-explain-dark-matter
 |access-date=7 January 2019
}}
</ref> However, that study assumed a monochromatic distribution to represent the LIGO/Virgo mass range, which is inapplicable to the broadly [[platykurtic]] mass distribution suggested by subsequent [[James Webb Space Telescope]] observations.<ref>
{{cite journal
 |last1=Zumalacárregui |first1=Miguel
 |last2=Seljak         |first2=Uroš
 |title=Limits on Stellar-Mass Compact Objects as Dark Matter from Gravitational Lensing of Type Ia Supernovae
 |journal=[[Physical Review Letters]]
 |date=1 October 2018
 |volume=121 |issue=14 |page=141101
 |doi=10.1103/PhysRevLett.121.141101  |pmid=30339429
 |arxiv=1712.02240  |bibcode=2018PhRvL.121n1101Z
 |s2cid=53009603
 |url=https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.141101
 |access-date=17 August 2023
}}
</ref><ref name=jwst/>

The possibility that atom-sized primordial black holes account for a significant fraction of dark matter was ruled out by measurements of positron and electron fluxes outside the Sun's heliosphere by the [[Voyager 1|Voyager&nbsp;1]] spacecraft. Tiny black holes are theorized to emit [[Hawking radiation]]. However the detected fluxes were too low and did not have the expected energy spectrum, suggesting that tiny primordial black holes are not widespread enough to account for dark matter.<ref>
{{cite news
 |title=Aging Voyager&nbsp;1 spacecraft undermines idea that dark matter is tiny black holes
 |date=9 January 2019
 |journal=[[Science (journal)|Science]]
 |via=sciencemag.org
 |url=https://www.science.org/content/article/aging-voyager-1-spacecraft-undermines-idea-dark-matter-tiny-black-holes
 |access-date=10 January 2019
}}
</ref> Nonetheless, research and theories proposing dense dark matter accounts for dark matter continue as of 2018, including approaches to dark matter cooling,<ref>
{{cite news
 |first=Shannon |last=Hall
 |date=5 February 2018 
 |title=There could be entire stars and planets made out of dark matter
 |magazine=[[New Scientist]]
 |url=https://www.newscientist.com/article/2160305-there-could-be-entire-stars-and-planets-made-out-of-dark-matter/
}}
</ref><ref>
{{cite journal
 |last1=Buckley  |first1=Matthew R.
 |last2=Difranzo |first2=Anthony
 |year=2018
 |title=Collapsed dark matter structures
 |journal=[[Physical Review Letters]]
 |volume=120 |issue=5 |page=051102
 |arxiv=1707.03829  |bibcode=2018PhRvL.120e1102B
 |doi=10.1103/PhysRevLett.120.051102
 |pmid=29481169  |s2cid=3757868
}}
</ref> and the question remains unsettled. In 2019, the lack of microlensing effects in the observation of Andromeda suggests that tiny black holes do not exist.<ref>
{{cite journal
 |last=Niikura |first=Hiroko
 |date=1 April 2019
 |title=Microlensing constraints on primordial black holes with Subaru/HSC Andromeda observations
 |journal=[[Nature Astronomy]]
 |volume=3 |issue=6 |pages=524–534
 |doi=10.1038/s41550-019-0723-1  |s2cid=118986293 
 |bibcode=2019NatAs...3..524N  |arxiv=1701.02151 
}}
</ref>

However, there still exists a largely unconstrained mass range smaller than that which can be limited by optical microlensing observations, where primordial black holes may account for all dark matter.<ref>
{{cite journal
 |last1=Katz       |first1=Andrey        |last2=Kopp |first2=Joachim
 |last3=Sibiryakov |first3=Sergey        |last4=Xue  |first4=Wei
 |date=5 December 2018 
 |title=Femtolensing by dark matter revisited
 |journal=Journal of Cosmology and Astroparticle Physics
 |volume=2018 |issue=12 |page=005
 |doi=10.1088/1475-7516/2018/12/005  |issn=1475-7516
 |bibcode=2018JCAP...12..005K  |arxiv=1807.11495
 |s2cid=119215426
 |url=http://stacks.iop.org/1475-7516/2018/i=12/a=005?key=crossref.dcb1e788286d8d75d0c7e7f35fe588eb
}}
</ref><ref>
{{cite journal
 |last1=Montero-Camacho |first1=Paulo        |last2=Fang  |first2=Xiao
 |last3=Vasquez  |first3=Gabriel             |last4=Silva |first4=Makana
 |last5=Hirata   |first5=Christopher M.
 |date=23 August 2019
 |title=Revisiting constraints on asteroid-mass primordial black holes as dark matter candidates
 |journal=[[Journal of Cosmology and Astroparticle Physics]]
 |volume=2019 |issue=8 |page=031
 |doi=10.1088/1475-7516/2019/08/031
 |issn=1475-7516  |bibcode=2019JCAP...08..031M
 |arxiv=1906.05950  |s2cid=189897766
}}
</ref>

=== Free streaming length ===
Dark matter can be divided into ''cold'', ''warm'', and ''hot'' categories.<ref>{{cite book |first=Joseph |last=Silk |title=The Big Bang: Third Edition |chapter-url={{google books |plainurl=y |id=XLwe1lUmz5kC |page=82}} |date=2000 |publisher=Henry Holt and Company |isbn=978-0-8050-7256-3 |chapter=IX}}</ref> These categories refer to velocity rather than an actual temperature, indicating how far corresponding objects moved due to random motions in the early universe, before they slowed due to cosmic expansion – this is an important distance called the ''[[free streaming]] length'' (FSL). Primordial density fluctuations smaller than this length get washed out as particles spread from overdense to underdense regions, while larger fluctuations are unaffected; therefore this length sets a minimum scale for later structure formation.

The categories are set with respect to the size of a [[protogalaxy]] (an object that later evolves into a [[dwarf galaxy]]): Dark matter particles are classified as cold, warm, or hot according to their FSL; much smaller (cold), similar to (warm), or much larger (hot) than a protogalaxy.<ref>{{cite book |last1=Bambi |first1=Cosimo |last2= D. Dolgov |first2=Alexandre |title=Introduction to Particle Cosmology |series=UNITEXT for Physics |year=2016 |language=English |publisher=Springer Berlin, Heidelberg | page=178 |doi=10.1007/978-3-662-48078-6 |isbn=978-3-662-48078-6}}</ref><ref>{{cite journal |first1=N. |last1=Vittorio |author2=J. Silk |date=1984 |journal=Astrophysical Journal Letters |volume=285 |pages=L39–L43 |title=Fine-scale anisotropy of the cosmic microwave background in a universe dominated by cold dark matter |doi=10.1086/184361 |bibcode=1984ApJ...285L..39V}}</ref><ref>{{Cite journal |first1=Masayuki |last1=Umemura |author2=Satoru Ikeuchi |title=Formation of Subgalactic Objects within Two-Component Dark Matter |date=1985 |journal=Astrophysical Journal |volume=299 |pages=583–592 |doi=10.1086/163726 |bibcode=1985ApJ...299..583U}}</ref> Mixtures of the above are also possible: a theory of [[mixed dark matter]] was popular in the mid-1990s, but was rejected following the discovery of [[dark energy]].{{Citation needed|date=April 2016}}

Cold dark matter leads to a bottom-up formation of structure with galaxies forming first and galaxy clusters at a latter stage, while hot dark matter would result in a top-down formation scenario with large matter aggregations forming early, later fragmenting into separate galaxies;{{Clarify|date=September 2018}} the latter is excluded by high-redshift galaxy observations.<ref name="bertone hooper silk" />

==== Fluctuation spectrum effects ====
These categories also correspond to [[fluctuation spectrum]] effects {{explain|reason=say what's fluctuating, and whether with time or distance|date=September 2021}} and the interval following the Big Bang at which each type became non-relativistic. Davis ''et al.'' wrote in 1985:<ref name="Davis-Efstath-etal-1985">{{cite journal |last1=Davis |first1=M. |author2=Efstathiou |first2=G. |author3=Frenk |first3=C. S. |author4=White |first4=S. D. M. |date=15 May 1985 |title=The evolution of large-scale structure in a universe dominated by cold dark matter |journal=Astrophysical Journal |volume=292 |pages=371–394 |bibcode=1985ApJ...292..371D |doi=10.1086/163168}}</ref>
{{blockquote|
Candidate particles can be grouped into three categories on the basis of their effect on the [[Mixed dark matter|fluctuation spectrum]] (Bond ''et al.'' 1983). If the dark matter is composed of abundant light particles which remain relativistic until shortly before recombination, then it may be termed "hot". The best candidate for hot dark matter is a neutrino ... A second possibility is for the dark matter particles to interact more weakly than neutrinos, to be less abundant, and to have a mass of order 1&nbsp;keV. Such particles are termed "warm dark matter", because they have lower thermal velocities than massive neutrinos ... there are at present few candidate particles which fit this description. [[Gravitino]]s and [[photino]]s have been suggested (Pagels and Primack 1982; Bond, Szalay and Turner 1982) ... Any particles which became nonrelativistic very early, and so were able to diffuse a negligible distance, are termed "cold" dark matter (CDM). There are many candidates for CDM including supersymmetric particles.<br/>
{{right| — Davis, Efstathiou, Frenk, & White (1985)<ref name=Davis-Efstath-etal-1985/>}} 
}}

==== Alternative definitions ====
Another approximate dividing line is warm dark matter became non-relativistic when the universe was approximately 1&nbsp;year old and 1&nbsp;millionth of its present size and in the [[radiation-dominated era]] (photons and neutrinos), with a photon temperature 2.7&nbsp;million&nbsp;Kelvins. Standard physical cosmology gives the [[particle horizon]] size as <math>2 c t</math> (speed of light multiplied by time) in the radiation-dominated era, thus 2&nbsp;light-years. A region of this size would expand to 2&nbsp;million light-years today (absent structure formation). The actual FSL is approximately 5&nbsp;times the above length, since it continues to grow slowly as particle velocities decrease inversely with the scale factor after they become non-relativistic. In this example the FSL would correspond to 10&nbsp;million light-years (or 3&nbsp;mega[[parsec]]s) today, around the size containing an average large galaxy.

The 2.7&nbsp;million&nbsp;[[Kelvin (unit)|Kelvin]] photon temperature gives a typical photon energy of 250&nbsp;[[electronvolt]], thereby setting a typical mass scale for warm dark matter: particles much more massive than this, such as GeV–TeV mass WIMPs, would become non-relativistic much earlier than one year after the Big Bang and thus have FSLs much smaller than a protogalaxy, making them cold. Conversely, much lighter particles, such as neutrinos with masses of only a few electronvolt, have FSLs much larger than a protogalaxy, thus qualifying them as hot.

==== Cold dark matter ====
{{Main|Cold dark matter}}
[[Cold dark matter]] offers the simplest explanation for most cosmological observations. It is dark matter composed of constituents with an FSL much smaller than a protogalaxy. This is the focus for dark matter research, as hot dark matter does not seem capable of supporting galaxy or galaxy cluster formation, and most particle candidates slowed early.

The constituents of cold dark matter are unknown. Possibilities range from large objects like MACHOs (such as black holes<ref name="Hawkins">{{Cite journal |last1=Hawkins |first1=M. R. S. |year=2011 |title=The case for primordial black holes as dark matter |journal=Monthly Notices of the Royal Astronomical Society |volume=415 |issue=3 |pages=2744–2757 |arxiv=1106.3875 |bibcode=2011MNRAS.415.2744H |doi=10.1111/j.1365-2966.2011.18890.x |s2cid=119261917 |doi-access=free}}</ref> and [[Preon star]]s<ref name="Phys.Lett.B616,1(2005">{{cite journal |last1=Hansson |first1=J. |last2=Sandin |first2=F. |year=2005 |title=Preon stars: a new class of cosmic compact objects |journal=[[Physics Letters B]] |volume=616 |issue=1–2 |pages=1–7 |arxiv=astro-ph/0410417 |doi=10.1016/j.physletb.2005.04.034 |bibcode=2005PhLB..616....1H |s2cid=119063004 }}</ref>) or [[Robust associations of massive baryonic objects|RAMBOs]] (such as clusters of brown dwarfs), to new particles such as WIMPs and [[axion]]s.

The 1997 [[DAMA/NaI]] experiment and its successor [[DAMA/LIBRA]] in 2013, claimed to directly detect dark matter particles passing through the Earth, but many researchers remain skeptical, as negative results from similar experiments seem incompatible with the DAMA results.

Many [[supersymmetry|supersymmetric]] models offer dark matter candidates in the form of the WIMPy [[Lightest Supersymmetric Particle]] (LSP).<ref>{{Cite journal |title=Supersymmetric dark matter |journal=Physics Reports |date=1 March 1996 |pages=195–373 |volume=267 |issue=5–6 |doi=10.1016/0370-1573(95)00058-5 |first1=Gerard |last1=Jungman |first2=Marc |last2=Kamionkowski |first3=Kim |last3=Griest |arxiv=hep-ph/9506380 |bibcode=1996PhR...267..195J|s2cid=119067698 }}</ref> Separately, heavy sterile neutrinos exist in non-supersymmetric extensions to the [[Standard Model|standard model]] which explain the small [[neutrino]] mass through the [[seesaw mechanism]].

==== Warm dark matter ====
{{Main|Warm dark matter}}
[[Warm dark matter]] comprises particles with an FSL comparable to the size of a protogalaxy. Predictions based on warm dark matter are similar to those for cold dark matter on large scales, but with less small-scale density perturbations. This reduces the predicted abundance of dwarf galaxies and may lead to lower density of dark matter in the central parts of large galaxies. Some researchers consider this a better fit to observations. A challenge for this model is the lack of particle candidates with the required mass ≈&nbsp;300&nbsp;eV to 3000&nbsp;eV.{{Citation needed|date=April 2016}}

No known particles can be categorized as warm dark matter. A postulated candidate is the [[sterile neutrino]]: a heavier, slower form of neutrino that does not interact through the [[Weak interaction|weak force]], unlike other neutrinos. Some modified gravity theories, such as [[scalar–tensor–vector gravity]], require "warm" dark matter to make their equations work.

==== Hot dark matter ====
{{Main|Hot dark matter}}
[[Hot dark matter]] consists of particles whose FSL is much larger than the size of a protogalaxy. The [[neutrino]] qualifies as such a particle. They were discovered independently, long before the hunt for dark matter: they were postulated in 1930, and [[Cowan–Reines neutrino experiment|detected in 1956]]. Neutrinos' [[Neutrino mass|mass]] is less than 10{{sup|−6}} that of an [[electron]]. Neutrinos interact with normal matter only via gravity and the [[Weak interaction|weak force]], making them difficult to detect (the weak force only works over a small distance, thus a neutrino triggers a weak force event only if it hits a nucleus head-on). This makes them "[[WISP (particle physics)|weakly interacting slender particles]]" ([[WISP (particle physics)|WISP]]s), as opposed to WIMPs.

The three known [[Flavour (particle physics)|flavours]] of neutrinos are the ''electron'', ''muon'', and ''tau''. Neutrinos oscillate among the flavours as they move. It is hard to determine an exact [[Upper and lower bounds|upper bound]] on the collective average mass of the three neutrinos. For example, if the average neutrino mass were over 50&nbsp;[[Electronvolt|eV]]/c{{sup|2}} (less than 10{{sup|−5}} of the mass of an electron), the universe would collapse.<ref>{{Cite journal |last1=Duan |first1=Huaiyu |last2=Fuller |first2=George M. |last3=Qian |first3=Yong-Zhong |date=2010-11-23 |title=Collective Neutrino Oscillations |url=https://www.annualreviews.org/doi/10.1146/annurev.nucl.012809.104524 |journal=Annual Review of Nuclear and Particle Science |language=en |volume=60 |issue=1 |pages=569–594 |doi=10.1146/annurev.nucl.012809.104524 |arxiv=1001.2799 |bibcode=2010ARNPS..60..569D |s2cid=118656162 |issn=0163-8998}}</ref> CMB data and other methods indicate that their average mass probably does not exceed 0.3&nbsp;eV/c{{sup|2}}. Thus, observed neutrinos cannot explain dark matter.<ref>
{{cite web
 |title=Neutrinos as dark matter
 |date=21 September 1998
 |publisher=Astro.ucla.edu
 |url=http://www.astro.ucla.edu/~wright/neutrinos.html
 |access-date=6 January 2011
}}
</ref>

Because galaxy-size density fluctuations get washed out by free-streaming, hot dark matter implies the first objects that can form are huge [[supercluster]]-size pancakes, which then fragment into galaxies. [[List of deep fields|Deep-field observations]] show instead that galaxies formed first, followed by clusters and superclusters as galaxies clump together.

===Dark matter aggregation and dense dark matter objects===
If dark matter is composed of weakly interacting particles, then an obvious question is whether it can form objects equivalent to [[planet]]s, [[star]]s, or [[black hole]]s. Historically, the answer has been it cannot,{{efn|
"One widely held belief about dark matter is it cannot cool off by radiating energy. If it could, then it might bunch together and create compact objects in the same way baryonic matter forms planets, stars, and galaxies. Observations so far suggest dark matter doesn't do that – it resides only in diffuse halos ... As a result, it is extremely unlikely there are very dense objects like stars made out of entirely (or even mostly) {{nobr|dark matter." — Buckley & Difranzo (2018)<ref name=curio/>}}
}}<ref name=curio>
{{cite journal
 |last1=Buckley |first1=Matthew R.
 |last2=Difranzo |first2=Anthony
 |date=1 February 2018
 |title=Synopsis: A way to cool dark matter
 |journal=[[Physical Review Letters]]
 |volume=120  |issue=5  |page=051102
 |doi=10.1103/PhysRevLett.120.051102
 |bibcode=2018PhRvL.120e1102B  |pmid=29481169
 |arxiv=1707.03829  |s2cid=3757868
 |url=https://physics.aps.org/articles/v11/s15
 |archive-url=https://archive.today/20201026224145/https://physics.aps.org/articles/v11/s15
 |archive-date=26 October 2020
}}
</ref><ref name=cornell_ask>
{{cite web
 |title=Are there any dark stars or dark galaxies made of dark matter?
 |department=Ask an Astronomer
 |website=curious.astro.cornell.edu
 |publisher=[[Cornell University]]
 |url=http://curious.astro.cornell.edu/about-us/95-the-universe/galaxies/general-questions/508-are-there-any-dark-stars-or-dark-galaxies-made-of-dark-matter-advanced
 |archive-url=https://web.archive.org/web/20150302105015/http://curious.astro.cornell.edu/about-us/95-the-universe/galaxies/general-questions/508-are-there-any-dark-stars-or-dark-galaxies-made-of-dark-matter-advanced
 |archive-date=2 March 2015
}}
</ref><ref name=siegel>
{{cite magazine
 |author-link=Ethan Siegel |author=Siegel, Ethan
 |date=28 October 2016
 |title=Why doesn't dark matter form black holes?
 |magazine=[[Forbes (magazine)|Forbes]]
 |url=https://www.forbes.com/sites/startswithabang/2016/10/28/why-doesnt-dark-matter-form-black-holes/#4e5014943de1
}}
</ref> because of two factors:
; It lacks an efficient means to lose energy<ref name=curio/> 
: Ordinary matter forms dense objects because it has numerous ways to lose energy. Losing energy would be essential for object formation, because a particle that gains energy during compaction or falling "inward" under gravity, and cannot lose it any other way, will heat up and increase [[velocity]] and [[momentum]]. Dark matter appears to lack a means to lose energy, simply because it is not capable of interacting strongly in other ways except through gravity. The [[virial theorem]] suggests that such a particle would not stay bound to the gradually forming object – as the object began to form and compact, the dark matter particles within it would speed up and tend to escape.
; It lacks a diversity of interactions needed to form structures<ref name=siegel/>
: Ordinary matter interacts in many different ways, which allows the matter to form more complex structures. For example, stars form through gravity, but the particles within them interact and can emit energy in the form of [[neutrino]]s and [[electromagnetic radiation]] through [[nuclear fusion|fusion]] when they become energetic enough. [[Proton]]s and [[neutron]]s can bind via the [[strong interaction]] and then form [[atom]]s with [[electron]]s largely through [[electromagnetic interaction]]. There is no evidence that dark matter is capable of such a wide variety of interactions, since it seems to only interact through gravity (and possibly through some means no stronger than the [[weak interaction]], although until dark matter is better understood, this is only speculation).

However, there are theories of [[atomic dark matter]] similar to normal matter that overcome these problems.<ref name=MirrorStars/>

== Detection of dark matter particles ==
If dark matter is made up of subatomic particles, then millions, possibly billions, of such particles must pass through every square centimeter of the Earth each second.<ref name="Gaitskell 2004">{{cite journal |last=Gaitskell |first=Richard J. |s2cid=11316578 |title=Direct Detection of Dark Matter |journal=[[Annual Review of Nuclear and Particle Science]] |volume=54 |pages=315–359 |bibcode=2004ARNPS..54..315G |date=2004 |doi=10.1146/annurev.nucl.54.070103.181244|doi-access=free}}</ref><ref name="Number per second">{{cite web |title=Neutralino Dark Matter |url=http://www.picassoexperiment.ca/dm_neutralino.php |access-date=26 December 2011}}
{{cite web |title=WIMPs and MACHOs |url=http://www.astro.caltech.edu/~george/ay20/eaa-wimps-machos.pdf |archive-url=https://web.archive.org/web/20060923123531/http://www.astro.caltech.edu/~george/ay20/eaa-wimps-machos.pdf |archive-date=2006-09-23 |url-status=live |last=Griest |first=Kim |access-date=26 December 2011}}</ref> Many experiments aim to test this hypothesis. Although WIMPs have been the main search candidates,<ref name="bertone hooper silk" /> [[axion]]s have drawn renewed attention, with the [[Axion Dark Matter Experiment]] (ADMX) searches for axions and many more planned in the future.<ref name="Chadha-Day et al">{{cite journal |author1=Chadha-Day |first=Francesca |author2=Ellis |first2=John |author3=Marsh |first3=David J. E. |date=23 February 2022 |title=Axion dark matter: What is it and why now? |journal=Science Advances |volume=8 |issue=8 |pages=eabj3618 |arxiv=2105.01406 |bibcode=2022SciA....8J3618C |doi=10.1126/sciadv.abj3618 |pmc=8865781 |pmid=35196098}}</ref> Another candidate is heavy [[hidden sector]] particles which only interact with ordinary matter via gravity.

These experiments can be divided into two classes: direct detection experiments, which search for the scattering of dark matter particles off atomic nuclei within a detector; and indirect detection, which look for the products of dark matter particle annihilations or decays.<ref name="bertone merritt" />

=== Direct detection ===
{{Further|Weakly interacting massive particle#Direct detection}}
{{Main|Direct detection of dark matter}}
Direct detection experiments aim to observe low-energy recoils (typically a few [[keV]]s) of nuclei induced by interactions with particles of dark matter, which (in theory) are passing through the Earth. After such a recoil, the nucleus will emit energy in the form of [[scintillation (physics)|scintillation]] light or [[phonon]]s as they pass through sensitive detection apparatus. To do so effectively, it is crucial to maintain an extremely low background, which is the reason why such experiments typically operate deep underground, where interference from [[cosmic ray]]s is minimized. Examples of underground laboratories with direct detection experiments include the [[Stawell Underground Physics Laboratory|Stawell mine]], the [[Soudan mine]], the [[SNOLAB]] underground laboratory at [[Greater Sudbury|Sudbury]], the [[Gran Sasso National Laboratory]], the [[Canfranc Underground Laboratory]], the [[Boulby Underground Laboratory]], the [[Deep Underground Science and Engineering Laboratory]] and the [[China Jinping Underground Laboratory]].

These experiments mostly use either cryogenic or noble liquid detector technologies. Cryogenic detectors operating at temperatures below 100&nbsp;mK, detect the heat produced when a particle hits an atom in a crystal absorber such as [[germanium]]. [[Noble gas|Noble liquid]] detectors detect [[scintillation (physics)|scintillation]] produced by a particle collision in liquid [[xenon]] or [[argon]]. Cryogenic detector experiments include such projects as [[Cryogenic Dark Matter Search|CDMS]], [[Cryogenic Rare Event Search with Superconducting Thermometers|CRESST]], [[EDELWEISS]], and [[European Underground Rare Event Calorimeter Array|EURECA]], while noble liquid experiments include [[LZ experiment|LZ]], [[XENON]], [[DEAP]], [[ArDM]], [[WIMP Argon Programme|WARP]], [[DarkSide (dark matter experiment)|DarkSide]], [[PandaX]], and LUX, the [[Large Underground Xenon experiment]]. Both of these techniques focus strongly on their ability to distinguish background particles (which predominantly scatter off electrons) from dark matter particles (that scatter off nuclei). Other experiments include [[SIMPLE (dark matter)|SIMPLE]] and [[PICASSO (dark matter)|PICASSO]], which use alternative methods in their attempts to detect dark matter.

Currently there has been no well-established claim of dark matter detection from a direct detection experiment, leading instead to strong upper limits on the mass and interaction cross section with nucleons of such dark matter particles.<ref>{{cite journal |last1=Drees |first1=M. |last2=Gerbier |first2=G. |title=Dark Matter |journal=Chin. Phys. C |date=2015 |volume=38 |page=090001 |url=http://pdg.lbl.gov/2015/reviews/rpp2015-rev-dark-matter.pdf |archive-url=https://web.archive.org/web/20160722093442/http://pdg.lbl.gov/2015/reviews/rpp2015-rev-dark-matter.pdf |archive-date=2016-07-22 |url-status=live}}</ref> The [[DAMA/NaI]] and more recent [[DAMA/LIBRA]] experimental collaborations have detected an annual modulation in the rate of events in their detectors,<ref>{{Cite journal |last1=Bernabei |first1=R. |last2=Belli |first2=P. |last3=Cappella |first3=F. |last4=Cerulli |first4=R. |last5=Dai |first5=C. J. |last6=d'Angelo |first6=A. |last7=He |first7=H. L. |last8=Incicchitti |first8=A. |last9=Kuang |first9=H. H. |last10=Ma |first10=J. M. |last11=Montecchia |first11=F. |last12=Nozzoli |first12=F. |last13=Prosperi |first13=D. |last14=Sheng |first14=X. D. |last15=Ye |first15=Z. P. |display-authors=6 |year=2008 |title=First results from DAMA/LIBRA and the combined results with DAMA/NaI |journal=Eur. Phys. J. C |volume=56 |issue=3 |pages=333–355 |arxiv=0804.2741 |bibcode=2008EPJC...56..333B |doi=10.1140/epjc/s10052-008-0662-y |s2cid=14354488}}</ref><ref>{{Cite journal |doi=10.1103/PhysRevD.33.3495 |pmid=9956575 |author1=Drukier, A. |author2=Freese, K. |author3=Spergel, D. |title=Detecting Cold Dark Matter Candidates |journal=Physical Review D |volume=33 |issue=12 |pages=3495–3508 |date=1986 |bibcode=1986PhRvD..33.3495D}}</ref> which they claim is due to dark matter. This results from the expectation that as the Earth orbits the Sun, the velocity of the detector relative to the [[dark matter halo]] will vary by a small amount. This claim is so far unconfirmed and in contradiction with negative results from other experiments such as LUX, SuperCDMS<ref>{{cite journal |last1=Davis |first1=Jonathan H. |title=The past and future of light dark matter direct detection |journal=Int. J. Mod. Phys. A |year=2015 |volume=30 |issue=15 |page=1530038 |doi=10.1142/S0217751X15300380 |arxiv=1506.03924 |bibcode=2015IJMPA..3030038D |s2cid=119269304 }}</ref> and XENON100.<ref>{{cite journal |last1=Aprile |first1=E. |title=Search for electronic recoil event rate modulation with 4&nbsp;years of XENON100 data |journal=Phys. Rev. Lett. |year=2017 |volume=118 |issue=10 |page=101101 |doi=10.1103/PhysRevLett.118.101101 |pmid=28339273 |arxiv=1701.00769|bibcode=2017PhRvL.118j1101A |s2cid=206287497 }}</ref>

A special case of direct detection experiments covers those with directional sensitivity. This is a search strategy based on the motion of the Solar System around the [[Galactic Center]].<ref name="apssyn">{{cite news |last=Stonebraker |first=Alan |title=Synopsis: Dark-Matter Wind Sways through the Seasons |newspaper=Physics – Synopses |publisher=[[American Physical Society]] |date=3 January 2014 |doi=10.1103/PhysRevLett.112.011301 }}</ref><ref name="samlee">{{cite journal |last1=Lee |first1=Samuel K. |first2=Mariangela |last2=Lisanti |first3=Annika H.G.|last3=Peter |first4=Benjamin R.|last4=Safdi |doi=10.1103/PhysRevLett.112.011301 |arxiv=1308.1953 |title=Effect of Gravitational Focusing on Annual Modulation in Dark-Matter Direct-Detection Experiments |date=3 January 2014 |journal=Phys. Rev. Lett. |volume=112 |issue=1 |page=011301 [5 pages] |bibcode=2014PhRvL.112a1301L |pmid=24483881|s2cid=34109648 }}</ref><ref name="dmgsheff">{{cite news |last=The Dark Matter Group |title=An Introduction to Dark Matter |newspaper=Dark Matter Research |location=Sheffield |publisher=University of Sheffield |url=http://www.hep.shef.ac.uk/research/dm/intro.php |access-date=7 January 2014 |archive-date=29 July 2020 |archive-url=https://web.archive.org/web/20200729020742/http://www.hep.shef.ac.uk/research/dm/intro.php  }}</ref><ref name="Kavli">{{cite news |quote=Scientists at Kavli MIT are working on ... a tool to track the movement of dark matter. |title=Blowing in the Wind |newspaper=Kavli News |location=Sheffield |publisher=[[Kavli Foundation (United States)|Kavli Foundation]] |url=http://www.kavlifoundation.org/science-spotlights/blowing-wind |access-date=7 January 2014 |archive-date=7 October 2020 |archive-url=https://web.archive.org/web/20201007192326/http://www.kavlifoundation.org/science-spotlights/blowing-wind |url-status=dead }}</ref> A low-pressure [[time projection chamber]] makes it possible to access information on recoiling tracks and constrain WIMP-nucleus kinematics. WIMPs coming from the direction in which the Sun travels (approximately towards [[Cygnus (constellation)|Cygnus]]) may then be separated from background, which should be isotropic. Directional dark matter experiments include [[Dark Matter Time Projection Chamber|DMTPC]], [[Directional Recoil Identification From Tracks|DRIFT]], Newage and MIMAC.

=== Indirect detection ===
{{main|Indirect detection of dark matter}}
[[File:Collage of six cluster collisions with dark matter maps.jpg|thumb|upright=2|Collage of six cluster collisions with dark matter maps. The clusters were observed in a study of how dark matter in clusters of galaxies behaves when the clusters collide.<ref>{{cite web |title=Dark matter even darker than once thought |url=http://www.spacetelescope.org/news/heic1506/ |access-date=16 June 2015 |website=Space Telescope Science Institute}}</ref>]]
[[File:Turning Black Holes into Dark Matter Labs.webm|thumb|Video about the potential [[Gamma-ray astronomy|gamma-ray detection]] of dark matter [[annihilation]] around [[supermassive black hole]]s. ''(Duration 0:03:13, also see file description.)'']]
Indirect detection experiments search for the products of the self-annihilation or decay of dark matter particles in outer space. For example, in regions of high dark matter density (e.g., the [[Galactic Center|centre of our galaxy]]) two dark matter particles could [[Annihilation|annihilate]] to produce [[gamma ray]]s or Standard Model particle–antiparticle pairs.<ref name="Bertone2010">{{cite book |first=Gianfranco |last=Bertone |title=Particle Dark Matter: Observations, Models and Searches |chapter-url=https://books.google.com/books?id=JkUgAwAAQBAJ&pg=PA83 |year=2010 |publisher=Cambridge University Press |pages=83–104 |chapter=Dark Matter at the Centers of Galaxies |arxiv=1001.3706 |isbn=978-0-521-76368-4 |bibcode=2010arXiv1001.3706M}}</ref> Alternatively, if a dark matter particle is unstable, it could decay into Standard Model (or other) particles. These processes could be detected indirectly through an excess of gamma rays, [[antiproton]]s or [[positron]]s emanating from high density regions in our galaxy or others.<ref>{{cite journal |last1=Ellis |first1=J. |last2=Flores |first2=R. A. |last3=Freese |first3=K. |last4=Ritz |first4=S. |last5=Seckel |first5=D. |last6=Silk |first6=J. |year=1988 |title=Cosmic ray constraints on the annihilations of relic particles in the galactic halo |url=https://cds.cern.ch/record/190709/files/198809398.pdf |url-status=live |journal=Physics Letters B |volume=214 |issue=3 |pages=403–412 |bibcode=1988PhLB..214..403E |doi=10.1016/0370-2693(88)91385-8 |archive-url=https://web.archive.org/web/20180728133226/https://cds.cern.ch/record/190709/files/198809398.pdf |archive-date=2018-07-28}}</ref> A major difficulty inherent in such searches is that various astrophysical sources can mimic the signal expected from dark matter, and so multiple signals are likely required for a conclusive discovery.<ref name="bertone hooper silk" /><ref name="bertone merritt" />

A few of the dark matter particles passing through the Sun or Earth may scatter off atoms and lose energy. Thus dark matter may accumulate at the center of these bodies, increasing the chance of collision/annihilation. This could produce a distinctive signal in the form of high-energy [[neutrino]]s.<ref>{{cite journal |doi=10.1016/0370-2693(86)90349-7 |author=Freese, K. |title=Can Scalar Neutrinos or Massive Dirac Neutrinos be the Missing Mass? |journal=Physics Letters B |volume=167 |issue=3 |pages=295–300 |date=1986 |bibcode=1986PhLB..167..295F}}</ref> Such a signal would be strong indirect proof of WIMP dark matter.<ref name="bertone hooper silk" /> High-energy neutrino telescopes such as [[Antarctic Muon And Neutrino Detector Array|AMANDA]], [[IceCube]] and [[ANTARES (telescope)|ANTARES]] are searching for this signal.<ref name=Randall_2015>{{cite book
 |first=Lisa |last=Randall
 |year=2015
 |title=Dark Matter and the Dinosaurs: The astounding interconnectedness of the Universe
 |publisher=Ecco / HarperCollins Publishers
 |location=New York, NY
 |isbn=978-0-06-232847-2
}}</ref>{{rp|298}} The detection by [[LIGO]] in [[GW150914|September 2015]] of gravitational waves opens the possibility of observing dark matter in a new way, particularly if it is in the form of [[primordial black hole]]s.<ref>{{cite magazine |url=https://www.newscientist.com/article/2077800-what-will-gravitational-waves-tell-us-about-the-universe |title=Surfing gravity's waves |magazine=New Scientist |first=Joshua |last=Sokol |display-authors=etal |issue=3061 |date=20 February 2016}}</ref><ref>{{cite web |publisher=Johns Hopkins University |title=Did gravitational wave detector find dark matter? |date=15 June 2016 |url=http://releases.jhu.edu/2016/06/15/did-gravitational-wave-detector-find-dark-matter/ |access-date=20 June 2015 |quote=While their existence has not been established with certainty, primordial black holes have in the past been suggested as a possible solution to the dark matter mystery. Because there is so little evidence of them, though, the primordial black hole–dark matter hypothesis has not gained a large following among scientists. The LIGO findings, however, raise the prospect anew, especially as the objects detected in that experiment conform to the mass predicted for dark matter. Predictions made by scientists in the past held conditions at the birth of the universe would produce many of these primordial black holes distributed approximately evenly in the universe, clustering in halos around galaxies. All this would make them good candidates for dark matter.}}</ref><ref>{{cite journal |last1=Bird |first1=Simeon |last2=Cholis |first2=Illian |year=2016 |title=Did LIGO detect dark matter? |journal=Physical Review Letters |volume=116 |issue=20 |page=201301 |doi=10.1103/PhysRevLett.116.201301 |pmid=27258861 |bibcode=2016PhRvL.116t1301B |arxiv=1603.00464|s2cid=23710177 }}</ref>

Many experimental searches have been undertaken to look for such emission from dark matter annihilation or decay, examples of which follow.

The [[Energetic Gamma Ray Experiment Telescope]] observed more gamma rays in 2008 than expected from the [[Milky Way]], but scientists concluded this was most likely due to incorrect estimation of the telescope's sensitivity.<ref>{{Cite journal |last1=Stecker |first1=F. W. |last2=Hunter |first2=S. |last3=Kniffen |first3=D. |year=2008 |title=The likely cause of the EGRET GeV anomaly and its implications |journal=Astroparticle Physics |volume=29 |issue=1 |pages=25–29 |arxiv=0705.4311 |bibcode=2008APh....29...25S |doi=10.1016/j.astropartphys.2007.11.002 |s2cid=15107441}}</ref>

The [[Fermi Gamma-ray Space Telescope]] is searching for similar gamma rays.<ref>{{Cite journal |title=The large area telescope on the Fermi Gamma-ray Space Telescope Mission |journal=Astrophysical Journal |volume=697 |issue=2 |year=2009 |pages=1071–1102 |doi=10.1088/0004-637X/697/2/1071 |arxiv=0902.1089 |first1=W.B. |last1=Atwood |last2=Abdo |first2=A.A. |last3=Ackermann |first3=M. |last4=Althouse |first4=W. |last5=Anderson |first5=B. |last6=Axelsson |first6=M. |last7=Baldini |first7=L. |last8=Ballet |first8=J. |last9=Band |first9=D.L. |display-authors=6 |bibcode=2009ApJ...697.1071A|s2cid=26361978 }}</ref> In 2009, an as yet unexplained surplus of gamma rays from the Milky Way's galactic center was found in Fermi data. This [[Galactic Center GeV excess]] might be due to dark matter annihilation or to a population of pulsars.<ref>{{Cite web |date=2019-11-12 |title=Physicists revive hunt for dark matter in the heart of the Milky Way |url=https://www.science.org/content/article/physicists-revive-hunt-dark-matter-heart-milky-way |access-date=2023-05-09 |website=www.science.org |language=en}}</ref> In April 2012, an analysis of previously available data from Fermi's [[Fermi LAT|Large Area Telescope]] instrument produced statistical evidence of a 130&nbsp;GeV signal in the gamma radiation coming from the center of the Milky Way.<ref>{{cite journal |doi=10.1088/1475-7516/2012/08/007 |last=Weniger |first=Christoph |title=A tentative gamma-ray line from dark matter annihilation at the Fermi Large Area Telescope |journal=Journal of Cosmology and Astroparticle Physics |issue=8 |date=2012 |arxiv=1204.2797 |volume=2012 |page=7 |bibcode=2012JCAP...08..007W|s2cid=119229841 }}</ref> WIMP annihilation was seen as the most probable explanation.<ref>{{cite web |url=http://physicsworld.com/cws/article/news/2012/apr/24/gamma-rays-hint-at-dark-matter |title=Gamma rays hint at dark matter |last1=Cartlidge |first1=Edwin |date=24 April 2012 |publisher=Institute of Physics |access-date=23 April 2013}}</ref>

At higher energies, [[IACT|ground-based gamma-ray telescopes]] have set limits on the annihilation of dark matter in [[dwarf spheroidal galaxy|dwarf spheroidal galaxies]]<ref>{{Cite journal |last1=Albert |first1=J. |last2=Aliu |first2=E. |last3=Anderhub |first3=H. |last4=Antoranz |first4=P. |last5=Backes |first5=M. |last6=Baixeras |first6=C. |last7=Barrio |first7=J.A. |last8=Bartko |first8=H. |last9=Bastieri |first9=D. |last10=Becker |first10=J.K. |last11=Bednarek |first11=W. |last12=Berger |first12=K. |last13=Bigongiari |first13=C. |last14=Biland |first14=A. |last15=Bock |first15=R.K. |last16=Bordas |first16=P. |last17=Bosch-Ramon |first17=V. |last18=Bretz |first18=T. |last19=Britvitch |first19=I. |last20=Camara |first20=M. |last21=Carmona |first21=E. |last22=Chilingarian |first22=A. |last23=Commichau |first23=S. |last24=Contreras |first24=J.L. |last25=Cortina |first25=J. |last26=Costado |first26=M.T. |last27=Curtef |first27=V. |last28=Danielyan |first28=V. |last29=Dazzi |first29=F. |last30=De Angelis |first30=A. |display-authors=6 |title=Upper Limit for γ-Ray Emission above 140&nbsp;GeV from the Dwarf Spheroidal Galaxy Draco |doi=10.1086/529135 |journal=The Astrophysical Journal |volume=679 |issue=1 |pages=428–431 |year=2008 |arxiv=0711.2574 |bibcode=2008ApJ...679..428A|s2cid=15324383 }}</ref> and in clusters of galaxies.<ref>{{Cite journal |last1=Aleksić |first1=J. |last2=Antonelli |first2=L.A. |last3=Antoranz |first3=P. |last4=Backes |first4=M. |last5=Baixeras |first5=C. |last6=Balestra |first6=S. |last7=Barrio |first7=J.A. |last8=Bastieri |first8=D. |last9=González |first9=J.B. |last10=Bednarek |first10=W. |last11=Berdyugin |first11=A. |last12=Berger |first12=K. |last13=Bernardini |first13=E. |last14=Biland |first14=A. |last15=Bock |first15=R.K. |last16=Bonnoli |first16=G. |last17=Bordas |first17=P. |last18=Tridon |first18=D.B. |last19=Bosch-Ramon |first19=V. |last20=Bose |first20=D. |last21=Braun |first21=I. |last22=Bretz |first22=T. |last23=Britzger |first23=D. |last24=Camara |first24=M. |last25=Carmona |first25=E. |last26=Carosi |first26=A. |last27=Colin |first27=P. |last28=Commichau |first28=S. |last29=Contreras |first29=J.L. |last30=Cortina |first30=J. |display-authors=6 |title=Magic Gamma-Ray Telescope observation of the Perseus Cluster of galaxies: Implications for cosmic rays, dark matter, and NGC&nbsp;1275 |doi=10.1088/0004-637X/710/1/634 |journal=The Astrophysical Journal |volume=710 |issue=1 |pages=634–647 |year=2010 |arxiv=0909.3267 |bibcode=2010ApJ...710..634A|s2cid=53120203 }}</ref>

The [[Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics|PAMELA]] experiment (launched in 2006) detected excess [[positron]]s. They could be from dark matter annihilation or from [[pulsar]]s. No excess [[antiproton]]s were observed.<ref>{{cite journal |last1=Adriani |first1=O. |last2=Barbarino |first2=G.C. |last3=Bazilevskaya |first3=G.A. |last4=Bellotti |first4=R. |last5=Boezio |first5=M. |last6=Bogomolov |first6=E.A. |last7=Bonechi |first7=L. |last8=Bongi |first8=M. |last9=Bonvicini |first9=V. |last10=Bottai |doi=10.1038/nature07942 |first10=S. |last11=Bruno |first11=A. |last12=Cafagna |first12=F. |last13=Campana |first13=D. |last14=Carlson |first14=P. |last15=Casolino |first15=M. |last16=Castellini |first16=G. |last17=De Pascale |first17=M.P. |last18=De Rosa |first18=G. |last19=De Simone |first19=N. |last20=Di Felice |first20=V. |last21=Galper |first21=A.M. |last22=Grishantseva |first22=L. |last23=Hofverberg |first23=P. |last24=Koldashov |first24=S.V. |last25=Krutkov |first25=S.Y. |last26=Kvashnin |first26=A.N. |last27=Leonov |first27=A. |last28=Malvezzi |first28=V. |last29=Marcelli |first29=L. |last30=Menn |first30=W. |display-authors=6 |title=An anomalous positron abundance in cosmic rays with energies 1.5–100&nbsp;GeV |journal=Nature |volume=458 |issue=7238 |pages=607–609 |year=2009 |pmid=19340076 |arxiv=0810.4995 |bibcode=2009Natur.458..607A|s2cid=11675154 }}</ref>

In 2013, results from the [[Alpha Magnetic Spectrometer]] on the [[International Space Station]] indicated excess high-energy [[cosmic ray]]s which could be due to dark matter annihilation.<ref name="APS-20130403">{{cite journal |title=First Result from the Alpha Magnetic Spectrometer on the International Space Station: Precision Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–350&nbsp;GeV |date=3 April 2013 |journal=[[Physical Review Letters]] |author=Aguilar, M. |collaboration=AMS Collaboration |doi=10.1103/PhysRevLett.110.141102 |bibcode=2013PhRvL.110n1102A |display-authors=etal |volume=110 |issue=14 |pmid=25166975 |page=141102|doi-access=free |hdl=1721.1/81241 |hdl-access=free }}</ref><ref name="AMS-20130403">{{cite web |title=First Result from the Alpha Magnetic Spectrometer Experiment |url=http://www.ams02.org/2013/04/first-results-from-the-alpha-magnetic-spectrometer-ams-experiment/ |date=3 April 2013 |author=AMS Collaboration |access-date=3 April 2013 |archive-url=https://web.archive.org/web/20130408185229/http://www.ams02.org/2013/04/first-results-from-the-alpha-magnetic-spectrometer-ams-experiment/ |archive-date=8 April 2013  }}</ref><ref name="AP-20130403">{{cite news |last1=Heilprin |first1=John |last2=Borenstein |first2=Seth |title=Scientists find hint of dark matter from cosmos |url=http://apnews.excite.com/article/20130403/DA5E6JAG3.html |date=3 April 2013 |agency=Associated Press |access-date=3 April 2013}}</ref><ref name="BBC-20130403">{{cite news |last=Amos |first=Jonathan |title=Alpha Magnetic Spectrometer zeroes in on dark matter |url=https://www.bbc.co.uk/news/science-environment-22016504 |date=3 April 2013 |work=BBC |access-date=3 April 2013}}</ref><ref name="NASA-20130403">{{cite web |last1=Perrotto |first1=Trent J. |last2=Byerly |first2=Josh |title=NASA TV Briefing Discusses Alpha Magnetic Spectrometer Results |url=http://www.nasa.gov/home/hqnews/2013/apr/HQ_M13-054_AMS_Findings_Briefing.html |date=2 April 2013 |website=NASA |access-date=3 April 2013}}</ref><ref name="NYT-20130403">{{cite news |last=Overbye |first=Dennis |title=New Clues to the Mystery of Dark Matter |url=https://www.nytimes.com/2013/04/04/science/space/new-clues-to-the-mystery-of-dark-matter.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2013/04/04/science/space/new-clues-to-the-mystery-of-dark-matter.html |archive-date=2022-01-01 |url-access=limited |date=3 April 2013 |work=The New York Times |access-date=3 April 2013}}{{cbignore}}</ref>

=== Collider searches for dark matter ===
An alternative approach to the detection of dark matter particles in nature is to produce them in a laboratory. Experiments with the [[Large Hadron Collider]] (LHC) may be able to detect dark matter particles produced in collisions of the LHC [[proton]] beams. Because a dark matter particle should have negligible interactions with normal visible matter, it may be detected indirectly as (large amounts of) missing energy and momentum that escape the detectors, provided other (non-negligible) collision products are detected.<ref name="kane watson">{{cite journal |author1=Kane, G. |author2=Watson, S. |title=Dark Matter and LHC:. what is the Connection? |journal=Modern Physics Letters A |year=2008 |volume=23 |pages=2103–2123 |doi=10.1142/S0217732308028314 |bibcode=2008MPLA...23.2103K |arxiv=0807.2244 |issue=26|s2cid=119286980 }}</ref> Constraints on dark matter also exist from the [[Large Electron–Positron Collider|LEP]] experiment using a similar principle, but probing the interaction of dark matter particles with electrons rather than quarks.<ref>{{cite journal |last1=Fox |first1=P. J. |last2=Harnik |first2=R. |last3=Kopp |first3=J. |last4=Tsai |first4=Y. |year=2011 |title=LEP Shines Light on Dark Matter |journal=Phys. Rev. D |volume=84 |issue=1 |page=014028 |arxiv=1103.0240 |bibcode=2011PhRvD..84a4028F |doi=10.1103/PhysRevD.84.014028 |s2cid=119226535}}</ref> Any discovery from collider searches must be corroborated by discoveries in the indirect or direct detection sectors to prove that the particle discovered is, in fact, dark matter.

== Alternative hypotheses ==
{{Further|Alternatives to general relativity}}

Because dark matter has not yet been identified, many other hypotheses have emerged aiming to explain the same observational phenomena without introducing a new unknown type of matter. The theory underpinning most observational evidence for dark matter, general relativity, is well-tested on solar system scales, but its validity on galactic or cosmological scales has not been well proven.<ref>{{cite book | author = Peebles, P. J. E.| date= December 2004<!--  | title=Testing general relativity on the scales of cosmology--> | arxiv= astro-ph/0410284|bibcode = 2005grg..conf..106P |doi = 10.1142/9789812701688_0010  | isbn = 978-981-256-424-5 | pages = 106–117 | chapter= Probing General Relativity on the Scales of Cosmology| title= General Relativity and Gravitation| s2cid= 1700265}}</ref> A suitable modification to general relativity can in principle conceivably eliminate the need for dark matter. The best-known theories of this class are [[Modified Newtonian dynamics|MOND]] and its relativistic generalization [[tensor–vector–scalar gravity]] (TeVeS),<ref>For a review, see: {{cite journal |title=The failures of the Standard Model of Cosmology require a new paradigm |author1=Kroupa, Pavel |display-authors=etal |journal=International Journal of Modern Physics D |date=December 2012 |volume=21 |issue=4 |page=1230003 |doi=10.1142/S0218271812300030 |arxiv=1301.3907 |bibcode=2012IJMPD..2130003K|s2cid=118461811 }}</ref> [[f(R) gravity]],<ref>For a review, see: {{cite journal |title=The dark matter problem from f(R) gravity viewpoint |author=Salvatore Capozziello |author2=Mariafelicia De Laurentis |journal=Annalen der Physik |date=October 2012 |volume=524 |issue=9–10 |page=545 |doi=10.1002/andp.201200109 |bibcode=2012AnP...524..545C |doi-access=free }}</ref> [[negative mass]], [[dark fluid]],<ref>{{cite web|url= https://www.ox.ac.uk/news/2018-12-05-bringing-balance-universe |title=Bringing balance to the Universe |date=5 December 2018 |publisher=University of Oxford}}</ref><ref>{{cite web|url= https://phys.org/news/2018-12-universe-theory-percent-cosmos.html |title=Bringing balance to the universe: New theory could explain missing 95 percent of the cosmos |publisher=Phys.Org}}</ref><ref name="Farnes">{{cite journal |last=Farnes |first=J. S. |year=2018 |title=A Unifying Theory of Dark Energy and Dark Matter: Negative Masses and Matter Creation within a Modified ΛCDM Framework |journal=Astronomy & Astrophysics |volume=620 |page=A92 |arxiv=1712.07962 |bibcode=2018A&A...620A..92F |doi=10.1051/0004-6361/201832898 |s2cid=53600834}}</ref> and [[entropic gravity]].<ref name="physorgnewtheory">{{cite news |title=New theory of gravity might explain dark matter |url=https://phys.org/news/2016-11-theory-gravity-dark.html |website=phys.org |date=November 2016}}</ref> [[Alternatives to general relativity|Alternative theories]] abound.<ref>{{cite journal |title=Alternatives to dark matter and dark energy |first=Phillip D. |last=Mannheim |journal=Progress in Particle and Nuclear Physics |volume=56 |issue=2 |pages=340–445 |doi=10.1016/j.ppnp.2005.08.001 |arxiv=astro-ph/0505266 |date=April 2006 |bibcode=2006PrPNP..56..340M |s2cid=14024934 }}</ref><ref>{{cite journal |title=Beyond the Cosmological Standard Model |first1=Austin |last1=Joyce |display-authors=etal |journal=Physics Reports |date=March 2015 |volume=568 |pages=1–98 |doi=10.1016/j.physrep.2014.12.002 |arxiv=1407.0059 |bibcode=2015PhR...568....1J|s2cid=119187526 }}</ref>

[[Primordial black hole]]s are considered candidates for components of dark matter.<ref name="Frampton"/><ref name="Lacki"/><ref>{{cite journal |last1=Villanueva-Domingo |first1=Pablo |last2=Mena |first2=Olga |last3=Palomares-Ruiz |first3=Sergio |title=A Brief Review on Primordial Black Holes as Dark Matter |journal=Frontiers in Astronomy and Space Sciences |date=2021 |volume=8 |page=87 |doi=10.3389/fspas.2021.681084 |arxiv=2103.12087 |bibcode=2021FrASS...8...87V |issn=2296-987X |doi-access=free }}</ref><ref>{{cite journal |last1=Green |first1=Anne M. |last2=Kavanagh |first2=Bradley J. |date=1 April 2021 |title=Primordial black holes as a dark matter candidate |url=https://iopscience.iop.org/article/10.1088/1361-6471/abc534 |journal=Journal of Physics G: Nuclear and Particle Physics |volume=48 |issue=4 |page=043001 |arxiv=2007.10722 |bibcode=2021JPhG...48d3001G |doi=10.1088/1361-6471/abc534 |issn=0954-3899 |s2cid=220666201 |access-date=17 August 2023}}</ref> Early constraints on primordial black holes as dark matter usually assumed most black holes would have similar or identical ("monochromatic") mass, which was disproven by LIGO/Virgo results.<ref name="Espinosa"/><ref name="Clesse"/><ref name="Kashlinsky"/>
In 2024, a review by [[Bernard Carr]] and colleagues concluded that primordial black holes forming in the [[quantum chromodynamics]] epoch prior to 10<sup>–5</sup> seconds after the Big Bang can explain most observations attributed to dark matter. Such black hole formation would result in an extended mass distribution today, "with a number of distinct bumps, the most prominent one being at around one solar mass."<ref name="Carr24" />

A problem with alternative hypotheses is that observational evidence for dark matter comes from so many independent approaches (see the "observational evidence" section above). Explaining any individual observation is possible but explaining all of them in the absence of dark matter is very difficult. Nonetheless, there have been some scattered successes for alternative hypotheses, such as a 2016 test of gravitational lensing in entropic gravity<ref>{{cite news |url=http://phys.org/news/2016-12-verlinde-theory-gravity.html |title=Verlinde's new theory of gravity passes first test |date=16 December 2016}}</ref><ref>{{cite journal |title=First test of Verlinde's theory of Emergent Gravity using Weak Gravitational Lensing measurements |first1=Margot M. |last1=Brouwer |display-authors=etal |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=466 |issue=3 |date=April 2017 |doi=10.1093/mnras/stw3192 |arxiv=1612.03034 |pages=2547–2559 |doi-access=free |bibcode=2017MNRAS.466.2547B|s2cid=18916375 }}</ref><ref>{{cite web |url=https://www.newscientist.com/article/2116446-first-test-of-rival-to-einsteins-gravity-kills-off-dark-matter/ |title=First test of rival to Einstein's gravity kills off dark matter |date=15 December 2016 |access-date=20 February 2017}}</ref> and a 2020 measurement of a unique MOND effect.<ref>{{cite web|url=https://www.sciencedaily.com/releases/2020/12/201216155158.htm|title=Unique prediction of 'modified gravity' challenges dark matter|publisher=ScienceDaily|date=16 December 2020|access-date=14 January 2021}}</ref><ref>{{cite journal|title=Testing the Strong Equivalence Principle: Detection of the External Field Effect in Rotationally Supported Galaxies|first1=Kyu-Hyun|last1=Chae|display-authors=et al|journal=[[Astrophysical Journal]]|volume=904|date=20 November 2020|issue=1|page=51|doi=10.3847/1538-4357/abbb96|arxiv=2009.11525|bibcode=2020ApJ...904...51C|s2cid=221879077 |doi-access=free }}</ref>

The prevailing opinion among most astrophysicists is that while modifications to general relativity can conceivably explain part of the observational evidence, there is probably enough data to conclude there must be some form of dark matter present in the universe.<ref name="CarrollTrialogue">{{cite web |author=Carroll |first=Sean |author-link=Sean M. Carroll |date=9 May 2012 |title=Dark matter vs. modified gravity: A trialogue |url=http://www.preposterousuniverse.com/blog/2012/05/09/dark-matter-vs-modified-gravity-a-trialogue/ |access-date=14 February 2017}}</ref>

== In popular culture ==
Dark matter regularly appears as a topic in hybrid periodicals that cover both factual scientific topics and science fiction,<ref>
{{cite journal
 |last=Cramer |first=John G.
 |date=1 July 2003 
 |title=LSST – the dark matter telescope
 |journal=[[Analog Science Fiction and Fact]]
 |volume=123 |issue=7/8 |page=96
 |issn=1059-2113 |id={{ProQuest|215342129}}
}} (Registration required)
</ref> and dark matter itself has been referred to as "the stuff of science fiction".<ref>
{{cite news
 |last=Ahern |first=James
 |date=16 February 2003
 |title=Space travel: Outdated goal
 |page=O&nbsp;02
 |work=[[The Record (Bergen County)|The Record]]
 |id={{ProQuest|425551312}}
}} (Registration required)
</ref>

Mention of dark matter is made in works of fiction. In such cases, it is usually attributed extraordinary physical or magical properties, thus becoming inconsistent with the hypothesized properties of dark matter in physics and cosmology. For example:
* Dark matter serves as a plot device in the ''[[The X-Files|X-Files]]'' episode "[[Soft Light (The X-Files)|Soft Light]]".<ref>
{{cite magazine
 |first=Grace |last=Halden
 |date=Spring 2015
 |title=Incandescent: Light bulbs and conspiracies
 |magazine=[[Dandelion (magazine)|Dandelion]]: Postgraduate Arts Journal and Research Network
 |volume=5 |number=2
 |doi=10.16995/ddl.318 |doi-access=free
}}
</ref> 
* A dark-matter-inspired substance known as ''"Dust"'' features prominently in [[Philip Pullman]]'s ''[[His Dark Materials]]'' trilogy.<ref>
{{cite book
 |first1=Mary |last1=Gribbin
 |first2=John |last2=Gribbin
 |year=2007
 |title=The Science of Philip Pullman's His Dark Materials
 |publisher=Random House Children's Books
 |isbn=978-0-375-83146-1
 |pages=15–30
}}
</ref>
* Beings made of dark matter are antagonists in [[Stephen Baxter (author)|Stephen Baxter]]'s [[Xeelee Sequence]].<ref>
{{cite journal
 |first=Andrew |last=Fraknoi
 |year=2019
 |title=Science fiction for scientists
 |journal=[[Nature Physics]]
 |volume=12 |issue=9 |pages=819–820
 |doi=10.1038/nphys3873 |s2cid=125376175
 |url=https://www.nature.com/articles/nphys3873
}}
</ref>

More broadly, the phrase "dark matter" is used metaphorically in fiction to evoke the unseen or invisible.<ref>
{{cite web
 |first=Adam |last=Frank |author-link=Adam Frank
 |date=9 February 2017
 |title=Dark matter is in our DNA
 |website=[[Nautilus Quarterly]]
 |url=https://nautil.us/dark-matter-is-in-our-dna-2-236435/
 |access-date=2022-12-11
}}
</ref>
<!-- Please do not add trivia about dark matter in popular culture to this section. Examples require secondary sources that provide context and indicate how those examples are significant. -->

==Gallery==
{{Gallery
|File:COSMOSDMmap2007.jpg|DM map by the [[Cosmic Evolution Survey]] (COSMOS) using the [[Hubble Space Telescope]] (2007)<ref>{{Cite web|title=First 3D map of the Universe's dark matter scaffolding|url=https://www.esa.int/Science_Exploration/Space_Science/First_3D_map_of_the_Universe_s_dark_matter_scaffolding|access-date=2021-11-23|website=www.esa.int|language=en}}</ref><ref>{{Cite journal|last1=Massey|first1=Richard|last2=Rhodes|first2=Jason|last3=Ellis|first3=Richard|last4=Scoville|first4=Nick|last5=Leauthaud|first5=Alexie|last6=Finoguenov|first6=Alexis|last7=Capak|first7=Peter|last8=Bacon|first8=David|last9=Aussel|first9=Hervé|last10=Kneib|first10=Jean-Paul|last11=Koekemoer|first11=Anton|date=January 2007|title=Dark matter maps reveal cosmic scaffolding|url=https://www.nature.com/articles/nature05497|journal=Nature|language=en|volume=445|issue=7125|pages=286–290|doi=10.1038/nature05497 |pmid=17206154|arxiv=astro-ph/0701594|bibcode=2007Natur.445..286M|s2cid=4429955|issn=1476-4687}}</ref>

|File:CFHTLenSDMmap2012.jpg|DM map by the CFHT Lensing Survey (CFHTLenS) using the [[Canada–France–Hawaii Telescope]] (2012)<ref>{{Cite web|title=News CFHT - Astronomers reach new frontiers of dark matter|url=https://www.cfht.hawaii.edu/en/news/CFHTLens/|access-date=2021-11-26|website=www.cfht.hawaii.edu}}</ref><ref>{{Cite journal|last1=Heymans|first1=Catherine|last2=Van Waerbeke|first2=Ludovic|last3=Miller|first3=Lance|last4=Erben|first4=Thomas|last5=Hildebrandt|first5=Hendrik|last6=Hoekstra|first6=Henk|last7=Kitching|first7=Thomas D.|last8=Mellier|first8=Yannick|last9=Simon|first9=Patrick|last10=Bonnett|first10=Christopher|last11=Coupon|first11=Jean|date=2012-11-21|title=CFHTLenS: the Canada–France–Hawaii Telescope Lensing Survey: CFHTLenS|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=427|issue=1|pages=146–166|arxiv=1210.0032|doi=10.1111/j.1365-2966.2012.21952.x|doi-access=free |s2cid=24731530}}</ref> (COSMOS map at the center)

|File:KiDSDMmap2015.gif|DM map by the Kilo-Degree Survey (KiDS) using the [[VLT Survey Telescope]] (2015)<ref>{{Cite web|title=KiDS|url=https://kids.strw.leidenuniv.nl/pr_july2015.php|access-date=2021-11-27|website=kids.strw.leidenuniv.nl}}</ref><ref>{{Cite journal|last1=Kuijken|first1=Konrad|last2=Heymans|first2=Catherine|last3=Hildebrandt|first3=Hendrik|last4=Nakajima|first4=Reiko|last5=Erben|first5=Thomas|last6=Jong|first6=Jelte T. A.|last7=Viola|first7=Massimo|last8=Choi|first8=Ami|last9=Hoekstra|first9=Henk|last10=Miller|first10=Lance|last11=van Uitert|first11=Edo|date=10 October 2015|title=Gravitational lensing analysis of the Kilo-Degree Survey|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=454|issue=4|pages=3500–3532|arxiv=1507.00738|doi=10.1093/mnras/stv2140|doi-access=free |issn=0035-8711}}</ref>

|File:HSCSDMmap2018.gif|DM map by the Hyper Suprime-Cam Survey (HSCS) using the [[Subaru Telescope]] (2018)<ref>{{Cite web|last=University|first=Carnegie Mellon|date=26 September 2018|title=Hyper Suprime-Cam Survey Maps Dark Matter in the Universe - News - Carnegie Mellon University|url=http://www.cmu.edu/news/stories/archives/2018/october/dark-matter-survey.html|url-status=live|website=www.cmu.edu|language=en|archive-url=https://web.archive.org/web/20200907194216/https://www.cmu.edu/news/stories/archives/2018/october/dark-matter-survey.html |archive-date=7 September 2020 }}</ref><ref>{{Cite journal|last1=Hikage|first1=Chiaki|last2=Oguri|first2=Masamune|last3=Hamana|first3=Takashi|last4=More|first4=Surhud|last5=Mandelbaum|first5=Rachel|last6=Takada|first6=Masahiro|last7=Köhlinger|first7=Fabian|last8=Miyatake|first8=Hironao|last9=Nishizawa|first9=Atsushi J|last10=Aihara|first10=Hiroaki|last11=Armstrong|first11=Robert|date=2019-04-01|title=Cosmology from cosmic shear power spectra with Subaru Hyper Suprime-Cam first-year data|url=https://academic.oup.com/pasj/article/doi/10.1093/pasj/psz010/5370019|journal=Publications of the Astronomical Society of Japan|language=en|volume=71|issue=2|page=43|arxiv=1809.09148|doi=10.1093/pasj/psz010|issn=0004-6264}}</ref>

|File:DESDMmap2021.png|DM map by the [[Dark Energy Survey]] (DES) using the [[Víctor M. Blanco Telescope]] (2021)<ref>{{Cite journal|last1=Jeffrey|first1=N|last2=Gatti|first2=M|last3=Chang|first3=C|last4=Whiteway|first4=L|last5=Demirbozan|first5=U|last6=Kovacs|first6=A|last7=Pollina|first7=G|last8=Bacon|first8=D|last9=Hamaus|first9=N|last10=Kacprzak|first10=T|last11=Lahav|first11=O|date=2021-06-25|title=Dark Energy Survey Year 3 results: Curved-sky weak lensing mass map reconstruction|url=https://academic.oup.com/mnras/article/505/3/4626/6287258|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=505|issue=3|pages=4626–4645|arxiv=2105.13539|doi=10.1093/mnras/stab1495|doi-access=free|issn=0035-8711}}</ref><ref>{{Cite journal|last=Castelvecchi|first=Davide|date=2021-05-28|title=The most detailed 3D map of the Universe ever made|url=http://www.nature.com/articles/d41586-021-01466-1|journal=Nature|language=en|pages=d41586–021–01466-1|doi=10.1038/d41586-021-01466-1|pmid=34050347|s2cid=235242965|issn=0028-0836}}</ref>
}}

== See also ==
{{columns-start}}
{{column}}
;Related theories
* {{annotated link|Dark energy}}
* {{annotated link|Conformal gravity}}
* [[Density wave theory]] – A theory in which waves of compressed gas, which move slower than the galaxy, maintain galaxy's structure
* {{annotated link|Entropic gravity}}
* {{annotated link|Dark radiation}}
* {{annotated link|Massive gravity}}
* {{annotated link|Unparticle physics}}
;Experiments
* {{annotated link|DEAP}}, a search apparatus
* {{annotated link|LZ experiment}}, large underground dark matter detector
* {{annotated link|Dark Matter Particle Explorer|abbreviation=DAMPE}}, a space mission
* {{annotated link|General antiparticle spectrometer}}
* {{annotated link|MultiDark}}, a research program
* {{annotated link|Illustris project}}, astrophysical simulations
* {{annotated link|Future Circular Collider}}, a particle accelerator research infrastructure
{{column}}
;Dark matter candidates
* {{annotated link|Feebly Interacting Particles}}
* {{annotated link|Light dark matter}}
* {{annotated link|Mirror matter}}
* {{annotated link|Exotic matter}}
* {{annotated link|Neutralino#Relationship to dark matter|Neutralino}}
* {{annotated link|Dark galaxy}}
* {{annotated link|Scalar field dark matter}}
* {{annotated link|Self-interacting dark matter}}
* {{annotated link|Weakly interacting massive particle|abbreviation=WIMP}}
* [[WISP (particle physics)|Weakly interacting slim particle]] (WISP){{snd}}Low-mass counterpart to WIMP
* {{annotated link|Strongly interacting massive particle|abbreviation=SIMP}}
* {{annotated link|Chameleon particle}}
;Other
* {{annotated link|Galactic Center GeV excess}}
* [[Luminiferous aether]] – A once theorized invisible and infinite material with no interaction with physical objects, used to explain how light could travel through a vacuum (now disproven)
{{columns-end}}

== Notes ==
{{notelist|1}}

== References ==
{{reflist|25em}}

==Further reading==
* {{cite magazine |first1=Sabine |last1=Hossenfelder |author1-link=Sabine Hossenfelder |first2=Stacy S. |last2=McGaugh |author2-link=Stacy S. McGaugh |title=Is dark matter real? |magazine=[[Scientific American]] |volume=319 |issue=2 |pages=36–43 |date=August 2018}}
* [[Rainer Weiss|Weiss, Rainer]], (July/August 2023) "The Dark Universe Comes into Focus" ''[[Scientific American]]'', vol. 329, no. 1, pp.&nbsp;7–8.
* {{cite arXiv|last1=Cirelli |first1=Marco |last2=Strumia |first2=Alessandro |last3=Zupan |first3=Jure |title=Dark Matter |date=2024 |class=hep-ph |eprint=2406.01705}}

== External links ==
{{Commons category|Dark matter}}
* {{cite AV media |url=http://video.ias.edu/the-fifth-element |title=Lecture on dark matter |author=Tremaine, Scott |publisher=IAS |medium=Video }}
* {{cite web |last1=Gray |first1=Meghan |title=Dark Matter |url=http://www.sixtysymbols.com/videos/darkmatter.htm |website=Sixty Symbols |editor-link=Brady Haran |editor=Haran, Brady |publisher=[[University of Nottingham]] |author2=Merrifield, Mike |author3=Copeland, Ed |date=2010}}

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[[Category:Celestial mechanics]]
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