http://phys.org/news/2015-04-potential-interacting-dark.html
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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.
Writing in the journal Monthly Notices of the Royal Astronomical Society today (Wednesday, April 15, 2015), the researchers said one dark matter clump appeared to be lagging behind the galaxy it surrounds.
They said the clump was currently offset from its galaxy by 5,000 light years (50,000 million million km)...
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.
