Dark Matter May Not be as 'Ghostly' as Previously Thought

Dark matter may not be nearly as ghostly or as "dark" as once thought. For the first time ever, scientists may have witnessed dark matter interacting with other dark matter in a way other than the through the force of gravity.

The researchers used the MUSE instrument on ESO's VLT in Chile along with images from Hubble in orbit. This allowed them to study the simultaneous collision of four galaxies in the galaxy cluster Abell 3827 and trace out where the mass lies within the system. Then, they compared the distribution of the dark matter with the positions of the galaxies.

Although dark matter can't be seen, the researchers could deduce its location by using gravitational lensing. The collision happened to take place in front of a much more distant, unrelated source, which means that 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.

All galaxies exist inside clumps of dark matter. Without the constraining effect of dark matter's gravity, galaxies like the Milky Way would fling themselves apart as they rotate. In this latest study, the researchers watched the four colliding galaxies and found that one dark matter clump appeared to be lagging behind the galaxy it surrounds.

The lag is actually predicted during collisions if dark matter interacts with itself through forces other than gravity. Since dark matter has never been observed interacting in any other way than through the force of gravity before, this finding is especially telling.

"We used to think that dark matter just sits around, minding its own business, except for its gravitational pull," said Richard Massey, one of the researchers, in a news release. "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."

The findings are published in the journal Monthly Notices of the Royal Astronomical Society.

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