Interstellar Gas

Some is very hot, like the Sun's corona
Some is luke warm (enough to ionize hydrogen)
Some is cool (neutral hydrogen - 21cm line)
Some is cold (molecular clouds)

Pressure = Density x Temperature
This gas is all at roughly the same pressure (otherwise it would expand or contract), so hotter gas is lower density.

Neutral hydrogen is detected with radio telescopes looking at the 21cm line of the hydrogen atom.

Dense, cold gas is detected with radio telescopes looking at molecular emission lines like CO

Temperature ~ 20 Kelvins
Density ~ 10⁶ atoms/cm³

Molecular gas clouds are often associated with regions of dense dust

Why are molecular clouds so cold?