Probing Matter at the Lowest Densities

Abstract
Between galaxies, matter densities are very low. This tenuous intergalactic matter can only be detected indirectly in the absorption spectra of bright sources such as quasars. In his Perspective, Miralda-Escudé charts recent progress in elucidating the properties of intergalactic matter through time. He highlights the report by Kriss et al., who have used the Far Ultraviolet Spectroscopic Explorer (FUSE) to chart the absorption spectrum of singly ionized helium, one of the two important species in these studies, at unprecedented resolution.