A team of scientists says it has found the most convincing evidence so far that the mysterious stuff known as dark matter really exists: a gigantic ring of invisible material left over from the ancient collision of galaxy clusters.
Researchers using the Hubble Space Telescope stumbled on the ring -- about 2.6 million light-years across -- while studying the galaxy cluster CI 0024+17, which is about 5 billion light-years from Earth.
Although scientists have discovered evidence of dark matter before, this would be the first time that dark matter has been found to assume a structure different from the surrounding visible matter.
"I was annoyed when I saw the ring because I thought it was an artifact, which would have implied a flaw in our data," said James Jee, associate research scientist at Johns Hopkins University in Baltimore, who announced the finding Tuesday at a news conference in Washington.
"I couldn't believe my result. It took more than a year to convince myself that the ring was real."
Because dark matter is invisible and nonreflective, it cannot be seen directly. The technique Jee and his team used to find the ring of dark matter is called gravitational lensing, in which light from a distant source is bent as it passes through very massive structures on its way to an observer.
Although no one has ever seen dark matter, most scientists are convinced it must exist.
Otherwise, galaxy clusters would fly apart instead of traveling together. That's because there's not enough mass in the clusters to produce the gravity necessary to keep all of the stars in place.
Most scientists think there is much more dark energy and dark matter than the matter we see.
Jee said he stumbled upon the ring of dark matter in late 2004, while studying the galaxy cluster in the constellation Pisces. Scientists think the cluster collided with another cluster from 1 billion to 2 billion years ago.
Jee described the dark matter ring as being like a ripple on the surface of a shallow pond that distorts the view of the pebbles on the bottom.
"The collision between the two galaxy clusters caused a ripple of dark matter that left distinct footprints in the shapes of the background galaxies," Jee said.
Previous observations of the Bullet Cluster -- 3.4 billion light-years away in the constellation Carina -- using the Hubble Space Telescope and the orbiting Chandra X-Ray Observatory showed dark matter being pulled apart by the hot gas of the two clusters. But the dark matter was imprinted with the shape of the visible matter.
The dark matter ring in CI 0024+17 does not follow any of the cluster's contours.
Computer simulations created by Jee's team show that when galaxy clusters collide, the dark matter falls to the center of the combined cluster and then washes back out. As it moves outward again, gravity slows it down, bunching it up like a freeway pileup.
"If it's real, it's incredibly exciting," said Caltech astronomer Richard Massey. But he added, "I've yet to be convinced."
For one thing, the kinds of observations that Jee and his team made are based on an extremely subtle analysis of data from the Hubble Space Telescope. Small imperfections can yield flawed results, he said.
Massey added that this galaxy cluster has a history of what he called "bad P.R."
"A lot of outrageous results have been claimed on this cluster" that turned out to be wrong, he said.
Massey said an independent observation of the same cluster with a different camera was needed to confirm the findings.
"I'd like to see more pictures," he said.