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Interstellar medium

matter and radiation in the space between the star systems in a galaxy, including ionic, atomic, or molecular gas, dust and cosmic rays, filling interstellar space and blending smoothly into the surrounding intergalactic space
Eagle nebula. Pillars made of dense cold gas are visible.

In astronomy, the interstellar medium (sometimes called the ISM) is the mix of plasma, gas, and dust in outer space between the stars of a galaxy. It is made mostly of ions, atoms, molecules, cosmic dust, and cosmic rays.

Various gases exist in outer space. Most common is hydrogen, followed by helium.