This composite image shows the galaxy cluster 1E 0657-56, also known as the "bullet cluster." This cluster was formed after the collision of two large clusters of galaxies, the most energetic event known in the universe since the Big Bang. NASA/CXC

"This provides the first direct proof that dark matter must exist," said Doug Clowe, a research astronomer at the University of Arizona.

Clowe and his colleagues used NASA's Chandra X-ray observatory, the Hubble Space Telescope and several ground-based observatories to examine the "bullet cluster," a clump of galaxies that formed over the last 100 million years from the violent collision of two smaller galactic clusters. The object gets its name from a bullet-shaped cloud of superhot gas on one of its sides.

Most of the visible mass in the bullet cluster is concentrated in that cloud and another near it. But using a technique known as gravitational lensing, Clowe and his colleagues show that the force of gravity is actually stronger in a part of the cluster that appears to be emptier.

They will publish their results in a future issue of Astrophysical Journal Letters.

"This is really exciting," said University of Chicago physicist Sean Carroll, adding that the observations demonstrate the existence of dark matter "beyond a reasonable doubt." Carroll was not involved in the research.

Astronomers have used dark matter for 70 years to explain various observations about the universe's behavior. They have shown that rotating spiral galaxies would fly apart if it were not for the gravitational pull of undetectable matter in addition to their stars. Other observations show that the expansion of the universe is being held back by a force greater than the gravitational pull of visible matter alone.

Though dark matter clearly provides the best explanation for such observations, Clowe said, "astronomers have long been in the slightly embarrassing position" of having to appeal to some mysterious, unobservable material in order to make things fit together.

Some physicists have even proposed that it isn't the amount and type of matter in the universe that needs to be adjusted, it's the law of gravity itself. They have suggested alternative theories that boost the strength of gravity on galactic and intergalactic scales in order to do away with the need for dark matter.

"It's always possible that there's some modification of gravity going on as well," Carroll said. "No matter what you do you're going to have dark matter."