The gas between the galaxies. In the early 1970s, the Uhuru X-ray satellite showed that rich clusters of galaxies contain gas at a temperature of about 108 K. In a typical cluster the mass of this gas is actually larger than the total mass of all the galaxies in the cluster. The absorption lines in distant quasars—the Lyman-α forest—show that there are also cool clouds of atomic hydrogen in intergalactic space. Our knowledge of the intergalactic medium, however, is seriously incomplete. For example, some astronomers argue that roughly half the normal matter in the Universe may be in the form of intergalactic gas with a temperature of about 106 K, but this gas would be virtually impossible to detect with current observational techniques.