Science throws new light on dark matter

The four galaxies in this cluster are involved in a massive collision taking place over a period of hundreds of millions of years. Photo / NASA

Dark matter may not be so dark after all, after scientists witnessed the mysterious cosmic entity interacting with the universe around it in an entirely new way.

Despite being thought to account for 85 per cent of matter in the universe, dark matter has never been seen directly by any scientific instruments. Its existence has only been inferred by its gravitational effects. But astronomers have now, for the first time, witnessed dark matter apparently "slowing down" after interaction with other dark matter - suggesting it is capable of engaging with a force other than gravity.

"We used to think that dark matter sits around, minding its own business," said Richard Massey of Durham University, who led the research. "But if it slowed down during this collision, this could be the first dynamical evidence that dark matter notices the world around it.

"Dark matter may not be completely dark after all."

Dark matter can only be detected indirectly by the way it bends the light of distant galaxies in a process known as gravitational lensing.

Exploiting this phenomenon, an international team of astronomers using the Hubble Space Telescope took images of a massive simultaneous collision in deep space between four distant galaxies.

The researchers found the dark matter associated with the colliding galaxies had become disconnected from this visible mass of colliding stars, suggesting it had come under the influence of a force other than gravity, probably by interacting with itself.

"The observations show that dark matter has ended up in a different place to the stars in the galaxy it was associated with," Massey said. "It has become offset in some way, and that's pretty unusual.

"We've been trying to think of other things that would cause this offset and there's nothing else we can think of that would have this effect other than dark matter interacting with itself. This is the first step in figuring out what dark matter is. To see it behaving in this way is the first positive thing we've seen dark matter do."

Dark secrets: what the picture shows

• The four galaxies in this cluster are involved in a massive collision taking place over a period of hundreds of millions of years. As the topmost of the four galaxies in the image begins to collide, it has left its dark matter trailing behind.

• The dark matter in this image is invisible, but it can be detected by the way it bends the light of an even more distant galaxy, in a process known as gravitational lensing, which has left a distorted image seen here as an arc of blue light just to the right of the cluster.

• The discovery that dark matter trails behind galaxies in this way suggests it is not perfectly dark after all.