Baryonic dark matter
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In astronomy and cosmology, baryonic dark matter is dark matter (matter that is undetectable by its emitted radiation, but whose presence can be inferred from gravitational effects on visible matter) composed of baryons, i.e. protons and neutrons and combinations of these, such as non-emitting ordinary atoms. Candidates for baryonic dark matter include non-luminous gas, Massive Astrophysical Compact Halo Objects (MACHOs: condensed objects such as black holes, neutron stars, white dwarfs, very faint stars, or non-luminous objects like planets), and brown dwarfs.
The total amount of baryonic dark matter can be inferred from Big Bang nucleosynthesis, and observations of the cosmic microwave background. Both indicate that the amount of baryonic dark matter is much smaller than the total amount of dark matter.
In the case of big bang nucleosynthesis, the problem is that large amounts of ordinary matter means a denser early universe, more efficient conversion of matter to helium-4 and less unburned deuterium that can remain. If one assumes that all of the dark matter in the universe consists of baryons, then there is far too much deuterium in the universe. This could be resolved if there were some means of generating deuterium, but large efforts in the 1970s failed to come up with plausible mechanisms for this to occur. For instance, MACHOs, which include, for example, brown dwarfs (balls of hydrogen and helium with masses less than 0.08 M☉ or 1.6×1029 kg), never begin nuclear fusion of hydrogen, but they do burn deuterium. Other possibilities that were examined include "Jupiters", which are similar to brown dwarfs but have masses 0.001 M☉ (2×1027 kg) and do not burn anything, and white dwarfs.[clarification needed]
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