Hot dark matter

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Hot dark matter is a form of dark matter which consists of particles that travel with ultrarelativistic velocities. The best example hot dark matter is the neutrino. Neutrinos have very small masses, and do not take part in two of the four fundamental forces, the electromagnetic interaction and the strong interaction. They do interact by the weak interaction, and gravity, but due to the feeble strength of these forces, they are difficult to detect. A number of projects, such as the Super-Kamiokande neutrino observatory, in Gifu, Japan are currently studying these neutrinos.

Dark matter is matter that cannot be detected by electromagnetic radiation, hence dark. It is postulated to exist to explain how clusters and superclusters of galaxies formed after the big bang. Data from galaxy rotation curves indicate that around 90% of the mass of a galaxy cannot be seen. It can only be detected by its gravitational effect.

Hot dark matter cannot explain how individual galaxies formed from the big bang. The microwave background radiation as measured by the COBE satellite is very smooth and fast moving particles cannot clump together on this small scale from such a smooth initial clumping. Due to theory,[which?] in order to explain small scale structure in the Universe, it is necessary to invoke cold or warm dark matter. Hot dark matter as the sole explanation of dark matter is no longer viable,[citation needed] therefore, it is nowadays considered only as part of a Mixed dark matter theory.

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