Hot Dark Matter (HDM) is a theoretical form of matter to explain the missing mass of the universe.
While there are two other candidate forms of matter - Warm Dark Matter and Cold Dark Matter - HDM distinguishes itself from the other two by being composed of elementary particles that travel at ultra relativistic energies; that is speeds approaching the speed of light (hence the Hot designation).
The particle considered most likely to be HDM is the neutrino.
Neutrinos meet the criteria for HDM in that they are very light - they are now thought to actually have a small amount of mass; previously it was thought that they were massless - and don't interact with other matter via the electromagnetic force or the strong force.
This means that neutrinos are dark (non-luminous) and that they don't interact strongly with other subatomic particles. These facts make them difficult to detect, but would still explain how it could interact gravitationally with other matter.
Is Hot Dark Matter the Likely Solution?
Given the weakness of the gravitational force compared to the other fundamental forces of nature coupled with the light mass of the neutrino it is nearly impossible to explain galaxy formation and the subsequent galactic rotation curves simply using the presence of neutrinos.
With their fast speed and small mass of the neutrino they do not pool together, which means that the apparently clumped dark matter formations in some star clusters and galaxies are unlikely to be these particles.
There are also at least three different types (or flavors) or neutrinos. One suggestion is that perhaps a bother type of neutrino exists, one with significantly more mass than those seen during the decay process of neutrons.
But having never been observed the existence of such particles is difficult to prove.
In either case it seems unlikely that neutrinos (and Hot Dark Matter in general) are the likely culprit to explain the missing mass of the Universe, at least as a whole. There are still theories that exist that use neutrinos as part of the solution; pushing more of a mixture of Hot, Warm and Cold Dark Matter as the solution.
