It could happen to anyone. Earlier this year, astronomers discovered a giant arc of matter more than three times the size of our Milky Way galaxy stretching across much of the distant cosmos.

At the time, the arc was believed to be among the largest detected objects in the universe. But after careful analysis, the discovery turned out to be a vast, glowing mirage.

Last January, Vahe Petrosian of Stanford University and C. Roger Lynds of the National Optical Astronomy Observatories in Tucson reported telescopic sightings of two giant luminous arcs. The astronomers did not attempt to explain the significance of the arcs, but they were reported to be the largest objects ever detected.

This week, however, the scientists announced that the objects actually were mirages, created by bizarre light-bending effects of a gravitational lens. Apparently, a massive field of gravity bent light waves from distant stars and, in the manner of any common lens, distorted the distant images.

A gravitational lens -- the name for the phenomenon -- was predicted by Einstein's General Theory of Relativity.

The information on the mass of the giant clusters may give astronomer's important clues about how much of the universe is undetected.

Astrophysicists believe that at least 90 percent of the universe consists of "dark matter," the invisible material that scientists hope will help explain the size, shape and future motions of the universe.

The amount of light-bending gravity found indicates that there is more mass in the distant cluster of stars than its light would suggest.

"Finding more than the predicted mass can help us understand how galaxies and clusters are formed," said Petrosian. "That is one of the outstanding problems in cosmology."