Lyman-alpha forest

The Lyman-alpha forest refers to a series of absorption lines in the spectra of distant quasars, primarily produced by intervening clouds of hydrogen gas in the intergalactic medium. When light from a quasar passes through these clouds, certain wavelengths of that light are absorbed due to the Lyman-alpha transition of hydrogen, which corresponds to a specific wavelength of 121.6 nanometers in the ultraviolet region of the spectrum.