An international team of astronomers has discovered what appears to be a galaxy composed entirely of dark matter. The galaxy, "visible" to radio telescopes, was first spotted by stargazers using the University of Manchester's Lovell telescope, and later confirmed by observers at the Arecibo telescope in Puerto Rico.

The astronomers have been scanning the skies for these dark galaxies - areas of matter that rotate like galaxies, but that don't give off any light - by studying how hydrogen atoms are distributed across the universe. During their search, they found an unexpectedly large quantity of hydrogen - a mass around 100m times that of the sun - in the Virgo cluster of galaxies, around 50m light-years away.

Dr Robert Minchin from Cardiff University is co-discoverer of the galaxy. He says that from the speed it is spinning, the team calculated that the galaxy, dubbed named VIRGOHI21, was a thousand times more massive than could be accounted for by the observed hydrogen atoms alone. The observational data from VIRGOHI21 so far suggests that it is a rotating flat disc of hydrogen, just like ordinary spiral galaxies.

"If it were an ordinary galaxy, then it should be quite bright and would be visible with a good amateur telescope," Minchin said.

Theory predicts that there is five times more dark matter than ordinary matter in the universe. The theory of galaxy formation also predicts that there ought to be more galaxies than we can see. These two factors have prompted researchers to hypothesise that there might be dark galaxies, hidden, unseen, throughout the universe. Scientists think these dark galaxies form when the material in a galaxy is not dense enough to form stars. ®

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