Astronomers say they have proven the existence of dark matter, an elusive form of matter believed to be much more abundant in the universe than the ordinary matter we can see. Detection came when they observed two massive clusters of galaxies colliding.

Galaxy cluster 1E 0657-56,  formed after the collision of two large clusters of galaxies

Until now, dark matter was just a hypothesis. It was first proposed more than 70 years ago to explain why some galaxies that moved through space at an unusually rapid speed did not fly apart. Scientists suggested that some unseen type of matter exerted enough gravitational pull to keep them together.

But University of Arizona astronomer Douglas Clowe says the notion of unseen matter has been discomforting.

"Astronomers have long been in the slightly embarrassing position of having to explain their observations using something that we didn't know actually existed," said Douglas Clowe.

But that has changed as the result of work Clowe and colleagues have done with the U.S. space agency's orbiting Chandra x-ray telescope. They have witnessed the collision of two massive clusters of galaxies, an impact so great that it has split normal and dark matter apart. This made it easier for them to detect dark matter by measuring its gravitational force apart from the gravity of the normal, observable matter in the stars and hot gases of the clusters.

"This provides the first direct proof that dark matter must exist and must make up the majority of the matter in the universe," he said.

The two galaxy clusters passed through each other at an incredible speed of 16 million kilometers per hour. As they did, the bulk of the luminous matter in the two clusters, which is in the form of hot gases, bumped into each other and slowed down. But the dark matter sailed ahead because it does not interact with normal matter the same way.

The researchers could tell the dark matter was there because the Hubble Space Telescope and large ground telescopes showed that its huge gravitational force bent light coming from distant objects behind it. This distortion, called gravitational lensing, magnified the objects, making them appear larger than if dark matter's gravity had been absent.

"The great news about this is that it is the once and for all the case that you can say dark matter does exist," said Sean Carroll.

Sean Carroll is a cosmologist at the University of Chicago who was not involved in the observations. He says particle physics laboratories around the world are trying to capture dark matter in an effort to determine its properties.

"So there absolutely is a new particle that physicists get to go out there and find," he said. "That's great news because it tells theorists what to think about - to think about models for dark matter - and experimentalists what to do to go out there and look for that particle."

Before this latest finding, some astronomers had proposed an alternative to dark matter. They suggested that ordinary matter's gravity might be stronger on the massive scale of galaxies and galaxy clusters. But Douglas Clowe says the new work shows that gravity's force is the same everywhere.

"There is simply no way to explain these observations without having an additional source of matter, which