Dark matter may not be completely "dark" after all as an international team of scientists has discovered the first potential signs of dark matter interacting with a force other than gravity.
The finding potentially rules out the standard theory of "Cold Dark Matter" where dark matter interacts only with gravity.
The team found that one dark matter clump appeared to be lagging behind the galaxy it surrounds.
The clump was currently offset from its galaxy by 5,000 light years (50,000 million million km) -- a distance that would take NASA's Voyager spacecraft 90 million years to travel.
Such an offset is 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 is thought to interact only with gravity, therefore making it invisible.
"We used to think that dark matter sits around, minding its own business. But if it slowed down during this collision, this could be the first dynamical evidence that dark matter notices the world around it," said lead researcher Richard Massey from the Durham University's institute for computational cosmology.
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.
"Our observation suggests that dark matter might be able to interact with more forces than just gravity. The dark matter could contain rich physics and potentially complex behaviour," explained team member professor Liliya Williams from the University of Minnesota.
Last month, Massey and colleagues published observations showing that dark matter interacted very little during 72 collisions between galaxy clusters (each containing up to 1,000 galaxies).
Taken together, the two results bracket the behaviour of dark matter for the first time.
"We are finally homing in dark matter from above and below - squeezing our knowledge from two directions. Dark matter, we are coming for you," Massey noted.
The team 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 the Earth.
Nobody knows what dark matter is but it is believed to make up about 85 percent of the universe's mass.
Without the constraining effect of its extra gravity, galaxies like our Milky Way would fling themselves apart as they spin.
The paper appeared in the journal Monthly Notices of the Royal Astronomical Society.
