First signs of self-interacting dark matter have suggested that it may not be as dark as previously believed.
Astronomers believe they might have observed the first potential signs of dark matter interacting with a force other than gravity.
An international team of scientists, led by researchers at Durham University, UK, made the discovery using the Hubble Space Telescope and the European Southern Observatory's Very Large Telescope to view the simultaneous collision of four distant galaxies at the centre of a galaxy cluster 1.3 billion light years away from Earth.
They said the clump was currently offset from its galaxy by 5,000 light years (50,000 million million km) - a distance it would take NASA's Voyager spacecraft 90 million years to travel.
Such an offset was predicted during collisions if dark matter interacts, even very slightly, with forces other than gravity.
Computer simulations show that the extra friction from the collision would make the dark matter slow down, and eventually lag behind.
Scientists believe that all galaxies exist inside clumps of dark matter - called "dark" because it was thought to interact only with gravity, therefore making it invisible.
In the latest study, the researchers were able to "see" the dark matter clump because of the distorting effect its mass has on the light from background galaxies - a technique called gravitational lensing.
The researchers added that their finding potentially rules out the standard theory of Cold Dark Matter, where dark matter interacts only with gravity.
The study is published in the journal Monthly Notices of the Royal Astronomical Society.
