Originally Posted by News @ Nature

A galaxy that is made almost entirely of dark matter has been discovered. It's the first galaxy found to have no stars at all, but it fits well with predictions made by astrophysicists about where the Universe's missing mass should be.

"We've thrown as many tests at it as we can, and it looks like a dark galaxy," says Robert Minchin from Cardiff University, UK, one of an international team of astronomers that made the find.

Dark matter betrays its presence by its gravitational pull: without dark matter to hold them together, rapidly rotating galaxies would simply fly apart. Scientists estimate that dark matter must be five times more abundant than normal matter in our Universe. It is likely to be made of relatively large subatomic particles that rarely interact with their surroundings, although these particles have never been identified.

In fact, more than 90% of our particular Galaxy's mass seems to be dark matter. The normal matter was pulled into stars, planets and dust clouds, but this doesn't seem to have happened in the dark galaxy. "What's bizarre is that the galaxy hasn't converted any gas into stars at all," says Neil Trentham, an astrophysicist from the University of Cambridge, UK.

The dark galaxy, named VIRGOHI21, is in the Virgo cluster, a large group of galaxies about 50 million light years away. It has roughly 10% of the mass of our own Galaxy, the Milky Way, "but it's not uncommonly small", says Minchin. The discovery will be reported in the Astrophysics Journal1.

Hydrogen shroud

The team's first clue came from the behaviour of the neutral hydrogen atoms that shroud this dark region of space. The researchers detected the characteristic radio-frequency signature of these atoms using the Lovell Telescope at the Jodrell Bank Observatory near Manchester, UK. They found that the hydrogen was swirling in exactly the same way as it would swirl around a normal, brightly lit galaxy.

At first, they assumed that they were simply looking at a dim, dwarf galaxy. But by watching how the hydrogen moved, the researchers were able to calculate that the mass of the galaxy is relatively large. However, normal matter packed that close should have ignited some stars.

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

He and his colleagues used the powerful Isaac Newton Telescope on La Palma in Spain's Canary Islands, to look for any scraps of visible light from the area; they found nothing. The most likely explanation is that the galaxy is made of dark matter, Minchin says.

Firm find

The inability to find dark galaxies has been a thorn in the side of theories about how dark matter shapes our Universe, which predict that there should be even more dark galaxies than visible ones. "The predictions were robust, but they hadn't been confirmed until now," says astrophysicist Ben Moore of the University of Zurich, Switzerland.

Scientists have also speculated that haloes of dark matter might be the gravitational seeds of galaxies, attracting enough normal matter to form stars (see 'Dark haloes pepper the Universe). "Finding this, and other dark galaxies in the future, will help us to understand how normal galaxies form," says Minchin.

The team now plans to use radio telescopes to hunt for more dark galaxies, says Minchin: "There could be many, many more of these things out there."