Interstellar medium is all the ordinary matter between the stars in a galaxy. It includes gases and tiny solid particles known as interstellar dust.
The average density of the gas in our home galaxy, the Milky Way, is only about one particle per cubic centimeter. This is roughly 1/1,000 the density of gas in the best vacuum that has been achieved in a laboratory. Most of the gas in the Milky Way consists of molecules, and single atoms, of hydrogen. The next most abundant element is helium, and other elements are much less plentiful.
Most of the dust consists of glassy silicate particles and graphitic matter. A silicate is a mineral containing silicon, oxygen, and one or more metallic elements. Graphitic matter contains graphite, a form of carbon. The dust particles come in a range of sizes. The vast majority of them are millions of times smaller than the dust that would be found on a desk at home or school.
Like ordinary smoke, interstellar dust blocks out visible light. Thus, the Milky Way’s interstellar medium first appeared to astronomers as oddly shaped dark patches in photographs taken in visible light. The patches appeared against the background “sea” of stars.
The interstellar medium is distributed unevenly within a galaxy. In the Milky Way and other spiral galaxies, the spiral arms have higher gas densities than the regions between the arms. Gigantic supernova remnants, masses of material thrown off by exploding stars, are filled with very hot, low-density gas.
Extremely hot stars heat the gas near them. As a result, this gas often glows in beautiful colors. New stars form when cold, especially dense regions of the interstellar medium collapse due to their own gravity.
