Most of the ISM is extremely diffuse. The average density is just one particle per cubic centimeter! (In comparison, a typical cubic centimeter of air at sea level contains about $3\times10^{19}$ — that’s 30,000,000,000,000,000,000 — particles.) Even the densest regions of the ISM contain only about 1,000,000 particles per cubic centimeter — which is almost as close as modern lab equipment can get to a perfect vacuum. One interesting effect of these incredibly low densities is that particles don’t encounter each other very often, which gives individual atoms enough time to undergo improbable energy transitions between collisions, producing “forbidden line” emission. These “forbidden” spectral lines are important to astronomers because they help reveal the composition of the ISM.