Using a host of telescopes, researchers focused on the collision between two galactic clusters. They found that most of the gravitational pull from the aftermath of the encounter comes from a relatively empty looking patch of sky, a strong suggestion that there is something more there than meets the eye.

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

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 100million years from the violent collision of two smaller galactic clusters. The object gets its name from a bullet-shaped cloud of super hot gas on one of its sides.

Bullet cluster

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.

Sean Carroll, a university of Chicago physicist, said: "This is really exciting", adding that the observations demonstrate the existence of dark matter "beyond a reasonable doubt".

Astronomers have used dark matter for 70 years to explain various observations about the universe's behaviour. 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.

Alternative theories

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 is not the amount and type of matter in the universe that needs to be adjusted, it is 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.

Carroll said: "It's always possible that there's some modification of gravity going on as well.

"No matter what you do you're going to have dark matter."