Dark matter is mysterious mass that exists in the universe and interacts with the rest of the universe via gravity, but apparently not by other means. It doesn't even seem to interact with other dark matter, except via gravity. Its origins and nature are largely unknown, although quasi-theoretical arguments seem to suggest that it consists of particles with masses on the same scale as fermions (e.g. electrons, protons, etc.). Neutrinos are technically dark matter, but there is a lot of matter that isn't neutrinos.

There is nothing in the universe that we know of that can have a negative temperature. That includes dark matter.

Dark matter produces an attractive force (gravity), while dark energy produces a repulsive force (antigravity). Together, they make up 96 percent of the universe—and we can't see either. Astronomers know dark matter exists because visible matter doesn't have enough gravitational muster to hold galaxies together.

I had no idea there could be anything colder than absolute zero, but indeed here's good scientific info about that:
absolute zero

Ultracold atoms pave way for negative-Kelvin materials.
... Absolute zero corresponds to the theoretical state in which particles have no energy at all, and higher temperatures correspond to higher average energies. ... However, by the 1950s, physicists working with more exotic systems began to realize that this isn't always true ... Another peculiarity of the sub-absolute-zero gas is that it mimics 'dark energy', the mysterious force that pushes the Universe to expand at an ever-faster rate against the inward pull of gravity.