The interstellar medium (ISM) is, as you say, the gas (and dust) in between the stars, within a galaxy. It consists of molecular, neutral and ionized gas, with densities ranging from $\sim 10^{-3}$ to $\sim 10^{6}$ particles per cm $^{3}$ and temperatures ranging from a few K up to 10,000 or 20,000 K. The molecular gas is the coldest and ... More @Wikipedia
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