We can see what happens at low energy densities, I’m not sure why you think it’s a mystery.

The funny thing is that standard human operating temperature is much closer to the coldest you can get than to the hottest. Absolute zero, when all thermal motion stops and it’s literally impossible to get any colder by definition, is only -273.15°C. We can reach it fairly easily, and we know that weird stuff does happen at low temperatures such as Bose-Einstein Condensates, but the universe really can’t cool a few orders of magnitude- there isn’t that much more cooling for it to do.