First signs of self-interacting dark matter observed

For the first time, scientists may have observed dark matter interacting with other dark matter, suggesting that the mysterious cosmic entity is capable of engaging with a force other than gravity.
The first hints about the nature of the elusive dark matter which makes up more than 80 per cent of our universe were picked up by observations of colliding galaxies made with European Southern Observatory's Very Large Telescope (VLT) and the NASA/ESA Hubble Space Telescope.
Using the MUSE instrument on ESO's VLT in Chile, along with images from Hubble in orbit, a team of astronomers studied the simultaneous collision of four galaxies in the galaxy cluster Abell 3827.

The team could trace out where the mass lies within the system and compare the distribution of the dark matter with the positions of the luminous galaxies.
Although dark matter cannot be seen, the team could deduce its location using a technique called gravitational lensing. The collision happened to take place directly in front of a much more distant, unrelated source.
The mass of dark matter around the colliding galaxies severely distorted spacetime, deviating the path of light rays coming from the distant background galaxy - and distorting its image into characteristic arc shapes.

The researchers observed the four colliding galaxies and found that one dark matter clump appeared to be lagging behind the galaxy it surrounds.

The dark matter is currently 5,000 light-years behind the galaxy - it would take NASA's Voyager spacecraft 90 million years to travel that far.
A lag between dark matter and its associated galaxy is predicted during collisions if dark matter interacts with itself, even very slightly, through forces other than gravity.
Dark matter has never before been observed interacting in any way other than through the force of gravity.
"We used to think that dark matter just sits around, minding its own business, except for its gravitational pull," said lead author Richard Massey at Durham University, UK.
"But if dark matter were being slowed down during this collision, it could be the first evidence for rich physics in the dark sector - the hidden Universe all around us," he said.

The researchers noted that more investigation will be needed into other effects that could also produce a lag. Similar observations of more galaxies, and computer simulations of galaxy collisions will need to be made.
"Our observation suggests that dark matter might interact with forces other than gravity, meaning we could rule out some key theories about what dark matter might be," said team member Liliya Williams of the University of Minnesota.