The original question was: What might be the state of matter in [interstellar space]. Physicist: Matter in deep space tends to take the form of gas. Generally, unless there’s a good reason to do otherwise, the individual atoms in space are traveling in every-which-way. The answers are: “yes it is” and “you have to think about why things cool off”. For example, “hot galaxies” are those with lots of randomly moving stars, and these galaxies literally evaporate (eject stars into inter-galactic space) and “cool” as a result.