Hot Dark Matter is one possible type of Dark Matter. It refers to non-baryonic subatomic particles that move at speeds very close to that of light. It's not "hot" in the traditional "it burned me" sense, but more in the sense that a particle moving so incredibly fast has a lot of energy.
The most likely form of Hot Dark Matter would be large numbers of Neutrinos. They move at speeds close to light, but don't interact with light or other electromagnetic radiation. They only interact with normal matter via Weak Interaction and Gravity, which makes them very difficult to detect.
Because of the math behind the suspected distribution of Hot Dark Matter in our galaxy, it's unlikely that all of it is Hot Dark Matter. Moving at speeds so close to light, it wouldn't be able to clump up as much as we think the Dark Matter has. But it could be one of several types of Dark Matter present in our universe.
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- Neutrinos can (theoretically) cause Nuclear Fission and break apart larger atoms into smaller ones. If so, and if the Dark Matter Halo includes a lot of Hot Dark Matter, it could pose a significant health risk to a ship trying to escape our galaxy.
- Neutrinos are believed to be a product of Supernova explosions. If much of the Dark Matter in the universe proves to be "Hot", that could mean that there's been a lot of explosions out at the edge of the galaxy, and that the Halo include a number of Neutron Stars or Black Holes.