The gaseous material between the stars, constituting several percent of the mass of stars in the Milky Way Galaxy. Being the reservoir from which new stars are born in the Galaxy, interstellar matter is of fundamental importance in understanding both the processes leading to the formation of stars, including the solar system, and ultimately to the origin of life in the universe.

Among the many ways in which interstellar matter is detected, perhaps the most familiar are attractive photographs of bright patches of emission-line or reflection nebulosity. However, these nebulae furnish an incomplete view of the large-scale distribution of material, because they depend on the proximity of one or more bright stars for their illumination. Radio observations of hydrogen, the dominant form of interstellar matter, reveal a widespread distribution throughout the thin disk of the Galaxy, with concentrations in the spiral arms. The disk is very thin (scale height 135 parsecs for the cold material; 1-pc is equal to 3.26 light-years, 1.92 × 10¹³ mi, or 3.09 × 10¹³ km) compared to its radial extent (the distance from the Sun to the galactic center is about 8000 pc, for example).

Mixed in with the gas are small solid particles, called dust grains, of characteristic radius 0.1 micrometer. Although by mass the grains constitute less than 1% of the material, they have a pronounced effect through the extinction (absorption and scattering) of starlight. Striking examples of this obscuration are the dark rifts seen in the Milky Way, and the Coalsack Nebula in the southern sky. On average, the density of matter is only 15 hydrogen atoms per cubic inch (1 hydrogen atom per cubic centimeter; in total, 2 × 10⁻²⁴ g · cm⁻³), but because of the long path lengths over which the material is sampled, this tenuous medium is detectable. Radio and optical observations of other spiral galaxies show a similar distribution of interstellar matter in the galactic plane.