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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 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 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) - a distance it 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.

Nobody knows what dark matter is, but it is believed to make up about 85 per cent of the Universe's mass.

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