Astronomers say they have found a vast ring of dark matter, the enigmatic substance that seems to make up most of the mass in the universe, encircling a cluster of galaxies five billion light-years away. If true, the discovery would mark the first time that dark matter has followed a much different shape than that of the visible matter it surrounds, highlighting the difference between the two types of matter.

At a NASA press conference today, the astronomers said the ring may have formed one billion to two billion years ago, after a collision between two galactic clusters caused the surrounding dark matter to ripple outward like a stone dropped in water. Some experts, however, say the ring may be nothing more than a kind of optical illusion.

Researchers do not know what dark matter is, but they invoke it to explain why galactic clusters have enough gravity to hold themselves together. Although dark matter does not reflect light—hence the name—astronomers can detect the way its gravity bends the light of distant stars behind it, an effect called gravitational lensing.

Prior studies of dark matter have found that galaxies and other visible matter float inside blobs of dark matter, like white chocolate chips in a dark cookie. So whatever shape the dark matter takes, such as a ring, it ought to carry galaxies with it.

A team using the Hubble Space Telescope found the invisible ring, which extends 2.6 million light-years across [see image above], while mapping the distribution of dark matter in the galaxy cluster CL 0024+17.

Astronomer M. James Jee of Johns Hopkins University says that at first he thought it was a glitch in the data. Now, he says, "we believe this is the strongest evidence yet for the existence of dark matter," adding that "no such kind of structure has been reported in the past."

A prior study suggested that CL 0024+17 had collided with another galactic cluster one billion to two billion years ago. Jee and colleagues simulated cluster collisions and found that the force of the event would slosh the surrounding dark matter outward into a ring, they report in a paper to appear June 20 in The Astrophysical Journal.

The result is interesting, but the evidence that the ring lacks visible matter is unconvincing so far, says astronomer Douglas Clowe of Ohio University in Athens, Ohio. "Even if the ring is real, it's not at all clear that it also is not seen in the galaxy distribution," he says.

Astronomer Dennis Zaritsky of the University of Arizona in Tucson notes that the ring could result from a chance alignment of dark matter lumps along Hubble's line of sight. There have been a few other reported claims of pure dark matter, Zaritsky says. "In the end, those things all went away."