Interstellar medium

In astronomy, the interstellar medium (sometimes called the ISM) is the tenuous gas and dust that fills the void between stellar systems in galaxies, that consisting mainly of huge clouds of ionized, neutral, or molecular hydrogen. This gas and dust not distributed uniformly in interstellar space, but displays significant variations in density, temperature, and ionisation state.
Various gases exist in outer space. Most common are hydrogen and helium. Interstellar cosmic dust makes nebulae.