interstellar medium

This term refers to the sparse matter filling the void between stars. It is a technical astronomical term used primarily in scientific discourse to describe the gas, dust, and cosmic rays that constitute the galactic environment outside of individual stellar systems.

As a mass noun representing a continuous substance or a specific region of space, it is treated as uncountable. It does not typically take a plural form in scientific literature, as it describes a singular, pervasive medium throughout a galaxy.