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Hot dark matter (HDM) is a form of dark matter which consists of particles that travel with ultrarelativistic velocities.

Dark matter is matter that does not interact with, and therefore 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 cosmic microwave background radiation as measured by the COBE satellite is very smooth and fast moving particles cannot form clumps as small as galaxies beginning from such a smooth initial state. Due to theory,[1] in order to explain small scale structure in the Universe, it is necessary to invoke cold dark matter (CDM) or warm dark matter (WDM). Hot dark matter as the sole explanation of dark matter is no longer viable,[1] therefore, it is nowadays considered only as part of a mixed dark matter (MDM) theory.

Neutrinos

The best example of a hot dark matter particle 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.
See also

Lambda-CDM model
Modified Newtonian Dynamics

References

Frenk, Carlos S.; White, Simon D. M. (2012). "Dark matter and cosmic structure". arXiv:1210.0544 [astro-ph.CO].