The absorption of light in the space between galaxies. It is too weak to detect between our own Galaxy and its near neighbours. However, high-redshift (and hence distant) quasars frequently reveal many Lyman-α absorption lines at redshifts smaller than that of the parent quasar. This is the Lyman-α forest, which arises from absorption by intervening clouds in intergalactic space.
Subjects: Astronomy and Astrophysics.