NASA's Chandra X-ray Observatory, the Hubble Space Telescope, the European Southern Observatory's Very Large Telescope and the Magellan optical telescopes were used to observe the violent collision between two large galaxy clusters 3 billion light years away. The force of the collision separated the dark and luminous matter, allowing a clear identification. Although scientists are yet to determine what form this mysterious dark matter may take, the observations are strong evidence that most of the matter in the universe is dark (Astrophysical J. and Astrophysical J. Letters to be published).

Dark matter was originally hypothesized to explain the abnormally high rotation speeds of galaxies, which would otherwise be torn apart if they did not contain hidden mass. It is fundamentally different from normal “luminous” matter such as stars as it is invisible to modern telescopes, giving off no light or heat, and seems to interact only through gravity.

However, some scientists do not believe that dark matter exists and have proposed alternative theories -- where gravity is stronger on intergalactic scales – to explain galactic dynamics. The new results are a blow to such theories. “Regardless of how one modifies gravity, it should still generally point to where most of the mass is,” says Maxim Markevitch at the Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, who was involved in the research. “If the only matter in this cluster was visible matter, the mass map would approximately follow the interstellar gas map. Instead, we found most of the mass elsewhere, exactly where if it were dominated by collisionless dark matter.”

Behind these observations lies a remarkable bullet-shaped cloud of hot gas produced by the collision of two clusters. As they cross at 10 million miles per hour, the luminous matter in each interacts with the other and slows down. But the dark matter does not interact at all, passing right through without disruption. This causes the dark matter to sail ahead, separating each cluster into two components: dark matter in the lead and luminous matter lagging behind.

To detect this separation, researchers compared x-ray images of the luminous matter with measurements of the cluster's total mass through gravitational lensing. This involves the observation of the distortion of light from background galaxies by the cluster's gravity -- the greater the distortion, the more massive the cluster. The team discovered four separate clumps of matter: two large clumps of dark matter speeding away from the collision, and two smaller clumps of luminous matter trailing behind, proving two types of matter exist.

The results have captured the imagination of the cosmology community. “This is an exciting discovery -- dark matter is not merely a trick of the light,” says Robert Caldwell, who is a cosmologist at Dartmouth College, in New Hampshire. “This result helps confirm we're on the right track in trying to solve the mystery of dark matter.”