The initial sighting of this invisible object came in 2000, from radio wave observations made using the Lovell telescope in Cheshire, England, which sketched a cloud of hydrogen atoms a million times the mass of the sun rotating in the Virgo cluster. Subsequent data from Puerto Rico's Arecibo radio telescope confirmed the existence of the object, dubbed VIRGOHI21. "From the speed it is spinning, we realized that VIRGOHI21 was a thousand times more massive than could be accounted for by the observed hydrogen atoms alone," commented co-discoverer Robert Minchin of Cardiff University. This suggests that abundant dark matter is lurking in the cloud. "If it was an ordinary galaxy, then it should be quite bright and would be visible with a good amateur telescope," he continued. Previous claims for dark galaxies have crumbled after observations using optical telescopes ultimately revealed resident stars. But scrutinizing the region using the Isaac Newton optical telescope in La Palma, Spain, revealed no visible stars.

Researchers have predicted the existence of unseen galaxies in recent years, based on indications that the universe contains far more matter than the visible variety can account for. Indeed, the astronomers involved in this new work noted that future surveys may well turn up many more dark galaxies. These findings will be published in the Astrophysical Journal.