8. What Temperature Is Dark Matter?
Given that we don't know anything about dark matter in the first place (apart from it has mass and we can't see it), puzzling about its temperature does rather feel like the modern physics equivalent of "How many angels can dance on the head of a pin?"
There is, however, reason behind the rather esoteric question, as knowing the answer might just help us figure out what it's made of.
There are two main theories here - HDM (hot dark matter) and CDM (cold dark matter) and it's not so much to do with their temperature as we're used to thinking about it, but the speed at which the particles move. Neutrinos are the main HDM candidate, but many scientists think that it would have to be mixed with CDM in order to form the clumps of matter we see today.
Lukewarm Dark Matter then? Doesn't sound very sci-fi