ESA's Infrared Space Observatory ISO has detected dust for the first time in the apparently empty space between the galaxies. German and Finnish astronomers made the discovery in the northern constellation of Coma Berenices, where more than 500 galaxies swarm together in the Coma Cluster. The intergalactic dust is concentrated towards the centre of the cluster.

In the past, astronomers have considered intergalactic space to be extremely clean. Except in the vast concentrations of stars, gas and dust that make up the galaxies themselves, the Universe was supposed to be filled only by very thin traces of invisible gas. ISO's detection of dust means that the Universe is less transparent than astronomers have assumed. Their cosmic window-pane is slightly dirty and large-scale inferences based on the brightnesses of distant galaxies and quasars may be affected.

Luminous Galaxies