The discovery of a big ball of hydrogen 50 million light-years from Earth may help unravel one of the thorniest problems in modern cosmology: Where is the missing dark matter in the universe?
Researchers at Cardiff University in Wales have measured a giant ball of hydrogen in the Virgo Cluster of Galaxies that they believe to be part of a much larger invisible galaxy of whirling debris.
If the finding stands up under the scrutiny of other cosmologists, it would be the best evidence yet that most of the matter in the universe is made up not of stars, but of a cold and invisible material known as dark matter.
"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. It's a really exciting discovery," said Jon Davies, a member of the Cardiff team that led the research. Other nations participating in the effort were France, Italy and Australia.
American scientists cautioned against making sweeping assumptions about the building blocks of the universe based on the European findings.
"It's probably premature to say what we're seeing here," said Stanford University physicist Roger Blandford.
"What's more significant is here we have a whole lot of gas that appears not to have formed stars," he said.
The finding was reported online Thursday and will be published next month in the Astrophysical Journal.
Based on a number of factors, including the accelerating speed at which galaxies are traveling, physicists have predicted that most of the mass of the universe is made up of unseen dark matter. Although various mathematical models have proved that dark matter must exist, the challenge has been to find clumps of it and to figure out what it is.
The team of researchers had been searching for a dark galaxy using radio waves, since a galaxy without stars could not be seen with a visible-light telescope. In 2000, the radio telescope at the University of Manchester in England detected a ball of invisible hydrogen with a mass equivalent to that of 100 million suns. The finding was confirmed by the Arecibo radio telescope in Puerto Rico.
The material in the ball was spinning at a high velocity, convincing the scientists that it was part of a larger spiral galaxy. The galaxy, they calculated, was a thousands times more massive than the gaseous ball of hydrogen, but cannot be seen because it has no stars.
The named the galaxy VIRGOHI21.
"If it were an ordinary galaxy, then it should be quite bright and would be visible with a good amateur telescope," said Robert Minchin, a member of the Cardiff team.
Other possible dark galaxies have been announced before, only to turn out to contain hidden stars when observed with high-powered telescopes. Others turned out to be the remains of colliding galaxies. The Cardiff team spent much of the five years since detecting the hydrogen ball eliminating other possibilities.
They found no stars and no traces of a galactic collision.
"As Sherlock Holmes said, 'When you have eliminated the impossible, whatever is left -- however improbable -- must be the truth,' " said Mike Disney of the Cardiff team.
The deeper question is: Why isn't VIRGOHI21 behaving like a normal galaxy? What is keeping the spinning gases and dust particles from gradually coalescing into fire-breathing stars?
Some American researchers expressed skepticism that VIRGOHI21 is really a galaxy at all. A galaxy has borders and is kept clumped together by the gravitational pull of the material in it. If this dark body has no boundaries, it could just be a drifting cloud of hydrogen with dark matter clumps hitching a ride.
"It's not really a dark matter galaxy, but a hydrogen cloud," said UCLA astronomer Ned Wright. "It's not really a bound system."
Other cosmologists were impressed by the team's effort to screen out other possibilities before announcing their finding.
"It looks like they've done their homework," said Lynn Cominsky, a professor of physics at Sonoma State University. "I think they've done a pretty careful job."
"It's good work," agreed Craig Hogan, vice provost for research at the University of Washington.
Even if VIRGOHI21 turns out not to be the first dark matter galaxy, it is clear that this is something unusual. That's enough to keep cosmologists interested for years to come.
"This has opened a new point of observation in space," Blandford said. "As we survey more of the sky, we will see if it's a fluke."