Interstellar matter, in astronomy, inclusive term for the gases and fine dust, known and inferred, that occupy the vast stretches of space between the stars in galaxies and between the galaxies themselves. Most of this material is not visible and can be detected only by study of its radio emissions. Space, on the average, is a vacuum more intense than any that can be created in the laboratory. In some locations, however, such as the spiral disk of galaxies like the Milky Way, the matter is relatively dense and may be seen as dark obscuring clouds or as clouds energised to glow or reflecting the light of nearby stars.
Interstellar matters are matters in a galaxy between the stars.
The grains may appear optically as bright reflection or dark nebulae. The rest of the matter is gas, mostly hydrogen and helium, but many molecules have been detected by radio astronomy techniques. Interstellar matter, in part, is primordial matter from the early history of the universe. It is also made up of matter ejected by novas or supernovas, consisting of elements, heavier than hydrogen, synthesised in stellar interiors. From this material, enriched in heavier elements, new stars are born.