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STRONG evidence for a massive galaxy totally devoid of stars has been found in the Virgo cluster, about 50 million light years away from Earth. If the existence of this “dark galaxy” is confirmed, it will vindicate the favoured theory of how galaxies form – and will present fresh puzzles to solve.

The new galaxy, which consists of a gigantic cloud of hydrogen gas and exotic dark matter, contains enough material to give birth to tens of millions of stars. Yet something is preventing this from happening. Such dark galaxies have been predicted, and could outnumber normal galaxies by as much as a hundred to one, but this is the first time anyone has confidently claimed to have seen one.

The discovery should come as a relief to astrophysicists developing theories of how galaxies form (see Graphic). “If there are no dark galaxies in the universe, then we must be missing an important piece of physics,” says Michael Merrifield of the University of Nottingham, UK.

For decades computer simulations have consistently predicted far more small galaxies than have been observed. For example, in our local group of galaxies there should be hundreds of dwarf galaxies, along with the gigantic Milky Way and Andromeda galaxies. Yet only 35 dwarf galaxies have been observed. One possibility is that these dwarfs exist as dark galaxies – starless clouds of hydrogen and dark matter. “The search for dark galaxies is crucial because there is a major disagreement between the theory of galaxy formation and observation,” says Riccardo Giovanelli of Cornell University in Ithaca, New York.

The theorists took heart last year with the announcement that the dwarf galaxy I Zwicky 18, situated just a cosmic stone’s throw from the Milky Way in a region where other galaxies are billions of years older, contained no stars older than 500,000 years (91av, 11 December 2004, p 16). Either the galaxy formed recently, or it has been hanging around as a dark galaxy for as long as 13 billion years.

Apart from such tantalising but inconclusive findings, no evidence has turned up until now. The most recent failure was last year’s HIPASS survey, which used the 64-metre Parkes radio telescope in New South Wales, Australia. “If we sweep in the corners of the data we might find one or two candidate objects, but nothing that makes for a convincing dark galaxy,” says Rachel Webster of the University of Melbourne, who led the study.

By definition, dark galaxies cannot be seen by optical telescopes. The only signs are radio waves with wavelengths of about 21 centimetres emitted by the hydrogen atoms that make up most of the gas in galaxies. Giovanelli says that HIPASS failed to find any dark galaxies because it was not sensitive enough.

In the latest survey, an international team led by Robert Minchin of Cardiff University, UK, used the sensitive 76-metre Lovell radio telescope at the University of Manchester’s Jodrell Bank Observatory to look for dark galaxies in the nearby Virgo cluster. There they found VIRGOHI21, a rotating cloud containing enough hydrogen gas to spawn 100 million stars like the sun and fill a small galaxy.

All previous possible dark galaxies have turned out to be duds: observations made using high-powered optical telescopes showed they contained stars after all. But when Minchin and his team used the 2.5-metre Isaac Newton optical telescope on the island of La Palma, Spain, they found no stars. “This is the first object we can be confident is a dark galaxy,” Minchin says.

But far from answering all the questions, VIRGOHI21 is throwing up a number of new ones. One concerns its mass. While the newly discovered galaxy is certainly dark, it may not be the dwarf that astrophysicists were hoping for.

“This is the first one we can be confident is a dark galaxy, all the previous candidates turned out to be duds”

If galaxies were made up only of ordinary matter, their speed of rotation would tear them apart. The extra mass needed to provide the gravitational pull that holds them together is generally thought to come from what is called dark matter. When Minchin’s team measured the speed of rotation of the hydrogen gas in VIRGOHI21, they found that it would have to contain about one-tenth of the dark matter of the Milky Way. But if that is so, it should also have a hundred times as much hydrogen gas as they actually detected. Far from being a dwarf, VIRGOHI21 seems to be a giant in its own right.

Merrifield says that the shortfall in the observed amount of hydrogen may mean that what Minchin and his team have seen is not a dark galaxy after all. “Their story doesn’t quite hang together, and I would speculate that they have been fooled by two passing hydrogen clouds.” The difference in speed as one passes the other would give the illusion of rotation, he says.

But Minchin is sticking to his guns. “There are so few known hydrogen clouds that to find two together would be extremely unlikely.” He thinks they may have underestimated the mass of hydrogen in the dark galaxy. If ultraviolet light from distant quasars were ionising a large proportion of the hydrogen atoms, the gas would be rendered invisible to radio telescopes.

Even if VIRGOHI21 is a mirage, other candidate dark galaxies are waiting in the wings. A new search with the world’s largest dish, the 305-metre Arecibo radio telescope in Puerto Rico, has turned up four in its first two weeks of operation. “We are 10 times more sensitive than the Jodrell Bank survey,” says Giovanelli, who is running the search. “We expect to find hundreds of dark galaxies within four or five years.”

If that happens, another group of theorists will have some explaining to do. Many of those who study stellar formation don’t see how stars could fail to form from the hydrogen cloud of a dark galaxy.

“With the world’s largest radio telescope, we expect to find hundreds of dark galaxies within four or five years”

Everyone agrees that for stars to form, the hydrogen atoms have to cool enough to form molecules, which then clump together under the influence of gravity to start forming stars. One way to stop this would be to somehow prevent the gas from cooling, but no one can suggest how.

This leaves Minchin unfazed. “Theory and observations often come into conflict,” he says. “Sometimes it is the observations that are wrong but mostly it’s the theories that are faulty.” He, for one, is convinced that VIRGOHI21 is a true dark galaxy.