A quasar absorption line is usually caused by the interstellar medium within or surrounding a galaxy. The quasar’s light goes through the medium, and the atoms in the medium absorb the quasar’s light at specific wavelengths.

Occasionally, the interstellar medium that causes a quasar absorption line is not associated with a single galaxy, but rather with a group or cluster of galaxies. It is also sometimes possible that the body of interstellar medium involved may be a large, free-floating intergalactic cloud. One particular kind of quasar absorber is called a Lyman-alpha cloud, which is a cloud of intergalactic gas that is much smaller than a typical galaxy and has almost no dust or heavy elements in it.