Hot dark matter moves at relativistic speeds. One candidate is the neutrino which is now known to have a small mass but insufficient to account for a significant amount of the missing mass.
MAssive Compact Halo Objects (MACHOs) and Weakly Interacting Massive Particles (WIMPs) are candidates for cold dark matter.
MACHOs include brown dwarfs, cooled white dwarfs, neutron stars, black holes and minor bodies such as planets, asteroids and comets. Measurements suggest that such material can account for no more than 20% of the Milky Way's dark matter halo. WIMPs have yet to be detected.
The search for dark matter continues but what of other possibilities? Is it correct to assume that stars in a galaxy, and galaxies in a cluster, are gravitationally bound? How would we know if they were not?
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