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," comments co-discoverer Robert Minchin of Cardiff University. This suggests that abundant dark matter is lurking in the cloud. "If it were an ordinary galaxy, then it should be quite bright and would be visible with a good amateur telescope," he continues. Previous claims for dark galaxies have crumbled after observations using optical telescopes ultimately revealed resident stars. But scrutinizing the region using the optical Isaac Newton telescope in La Palma, Spain, the team did not spot any such signs of the ordinary.
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 note that future surveys may well turn up many more dark galaxies. "The universe has all sorts of secrets still to reveal to us, but this shows that we are beginning to understand how to look at it in the right way," remarks team member Jon Davies, also at Cardiff University. "It's a really exciting discovery." A report detailing these findings will be published in the Astrophysical Journal.