the.com/interstellar medium
space isn't empty, it's just very thin soup with excellent lighting.
means the sparse gas and dust that fills the space between stars in a galaxy, mostly hydrogen with a dash of everything else.
from astronomers realized in the early 1900s that starlight was getting dimmed and reddened by something between us and distant stars, evidence of matter in the supposed void, later mapped through radio and absorption lines.
densityabout one atom per cubic centimeter, roughly
composition99 percent gas, 1 percent dust
star nurserydense clouds collapse to form new stars
recyclingdying stars enrich it with heavier elements