Interstellar Medium

From Slackerpedia Galactica

Smaller than an interstellar large, and much smaller than an interstellar extra large. Goes great with a French cruller or a Boston creme doughnut.

Or, the wispy stuff that floats between stars in a galaxy, and mainly spiral galaxies at that. Most galaxies are filled with very tenuous gas, mainly hydrogen, helium, and traces of other elements, and interstellar medium is a catch-all for anything in a galaxy that isn't a star (or dark matter) and lies outside any given solar system's outer reaches also known as the heliopause. It includes everything from super-hot, super-tenuous bubbles of gas blown up by supernovae, to cold, dark dust clouds from which form new stars.