The colour of matter: This astounding view of galaxy cluster MACS J0025.4-1222 demonstrates how ordinary matter and mysterious dark matter interact. The blue cloud-shaped parts flanking the centre show the position of dark matter, mapped by the Advanced Camera for Surveys (ACS) onboard the NASA/ESA Hubble Space Telescope. The pink middle indicates ordinary matter, charted by NASA's Chandra X-Ray Observatory.

Credit: NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara, USA), and S. Allen (Stanford University, USA).

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WASHINGTON DC: Space telescopes have captured images of a mammoth collision between two galaxy clusters that have shed new light into the universe's mysterious dark matter.

The images taken by the Hubble Space Telescope and Chandra X-ray Observatory show a clear separation between dark and ordinary, or baryonic, matter during the clash 5.7 billion light years from Earth, said the U.S. space agency, NASA.

Gravitational lensing

"It is in our view an important step forward to understanding the properties of the mysterious dark matter," said Marusa Bradac, a University of California, Santa Barbara researcher who leads the team that captured the collision.

The astronomers were able to differentiate between the two substances with a technique known as gravitational lensing, which infers the presence of dark matter by the effect its gravity has on light. Dark matter (detected by Hubble with the lensing technique) appears in blue in the image while ordinary matter, which is mostly in the form of hot gas (detected by Chandra), looks pink.

The experts theorised that as the two clusters of galaxies merged at speeds of millions of km/h, the hot gas in each cluster collided and slowed down, while the dark matter did not.

The separation between the two types of material in the images provides direct evidence for dark matter, the experts added, and supports the ideas that particles of baryonic- and dark matter interact with each other only very weakly or not all, apart from the pull of gravity.

"A very different kind of matter"

"Dark matter makes up five times more matter in the universe than ordinary matter. This study confirms that we are dealing with a very different kind of matter, unlike anything that we are made of," said Bradac.

The discovery independently confirms the 2006 discovery of another galactic collision known as the Bullet Cluster, which also showed a clear separation between dark and ordinary matter.

Around a fifth of the universe is believed to consist of dark matter, spreading out in mysterious filaments, sheets and clusters. But, with present technology, it cannot be seen directly. Its existence is perceived indirectly, through the gravitational pull it exerts on light.