article: The first signs of dark matter interacting with a force other than gravity may have been found. Until now it was thought that dark matter did not interact with anything other than gravity, earning it its ‘dark’ moniker and making its detection incredibly difficult. But the discovery of a ‘clump’ lagging behind a galaxy suggests that it may not be as oblivious to our universe as we thought. Durham University scientists studied a 'clump' of dark matter that appears to be lagging behind its galaxy - suggesting it interacts with itself. In this Hubble image, the lagging clump can be seen on the left of the central cluster. The distribution of dark matter in the cluster is shown with blue contour lines . Dark matter is confusing because it interacts with nothing - including itself. This is despite seeming to account for 85 per cent of the universe’s mass and having an observable effect on galaxies: without dark matter, our galaxies would ‘fall apart’ as they spin. Aside from gravity, though, dark matter seemed to just pass through the universe without interacting with any other mass, such as humans, planets or stars. Now an international team of researchers at Durham University has found a clump offset from a galaxy by 5,000 light-years, suggesting that is not the case. When physicists study the dynamics of galaxies and the movement of stars, they are confronted with a mystery. If they only take visible matter into account, their equations don't add up; the elements that can be observed are not sufficient to explain the rotation of objects and the existing gravitational forces. There is something missing. From this they deduced there must be an invisible kind of matter that does not interact with light but does, as a whole, interact by means of the gravitational force. Called 'dark matter', this substance appears to make up 85 per cent of the matter in the known universe. When it comes to the total energy in the universe, though, it's a different story. Astronomers have found that the total mass/energy content of the universe is split in the proportions 68 per cent dark energy, 27 per cent dark matter and 5 per cent 'normal' matter. Dark energy is an unknown force across the whole cosmos that seems to be accelerating the expansion of the universe. So the 85 per cent figure above relates only to the fraction of 'matter' that is dark. The discovery was made around one of the galaxies in the Abell 3827 cluster, 1.3 billion light-years away. Here, four galaxies are colliding around the centre of the cluster. The team used the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile, along with images from Hubble, to view Abell 3827. The offset is important because it is only possible if dark matter interacts with itself through forces other than gravity. If the particles collide with each other, the friction from the collisions would cause the dark matter to slow down, and fall behind the galaxy. The nature of that interaction is unknown; it could be caused by well-known effects or some unknown force. All that can be said at this point is that it is not gravity. But dark matter has never before been observed interacting in any way other than through the force of gravity - making the discovery of huge interest. ‘We used to think that dark matter just sits around, minding its own business, except for its gravitational pull,’ Dr Richard Massey from Durham University said. ‘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 added to MailOnline: 'This is the first time we've caught dark matter in the act of doing something interesting. 'Once the dark universe around us is allowed to interact at all, the possibilities for what it could get up to are rich and varied.' This image from the Nasa/Esa Hubble Space Telescope shows a zoomed out view of the galaxy cluster Abell 3827, 1.3 billion light-years away. The strange blue structures surrounding the central galaxies are gravitationally lensed views of a much more distant galaxy behind the cluster . 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 space-time, deviating the path of light rays coming from the distant background galaxy - and distorting its image into characteristic arc shapes. Our current understanding is that all galaxies exist inside clumps of dark matter, which is how they are able to survive without flinging themselves apart. The team used the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile, pictured, along with images from the Hubble Space Telescope, to make the discovery . 'We know that dark matter exists because of the way that it interacts gravitationally, helping to shape the universe, but we still know embarrassingly little about what dark matter actually is,' said team member Dr Liliya Williams of the University of Minnesota. '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.' The researchers note 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. This result follows on from recent findings from the team which observed 72 collisions between galaxy clusters and found that dark matter interacts very little with itself. This latest study follows research by the same team last month that found dark matter interacts with itself very little - but the fact that it interacts at all is very exciting, said Dr Massey. Shown is the galaxy cluster MACS J0416.1+2403 and its dark matter distribution, shown in blue, based on its gravitational interactions . The new work, however, concerns the motion of individual galaxies, rather than clusters of galaxies. Researchers said that the collision between these galaxies could have lasted longer than the collisions observed in the previous study - allowing the effects of even a tiny frictional force to build up over time and create a measurable lag. Taken together, the two results bracket the behaviour of dark matter for the first time. Dr Massey added: 'We are finally homing in on dark matter from above and below - squeezing our knowledge from two directions.'
highlights: Durham University scientists studied a 'clump' of dark matter .
  It was found to lag behind the galaxy it was associated with .
  This suggests dark matter particles can self-interact and slow down .
  It means dark matter may not be as oblivious to our universe as we thought .
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