Dark matter is a giant question mark looming over our knowledge of the Universe. There is more dark matter in the Universe than visible matter, but it is extremely elusive; it does not reflect, absorb or emit light, making it invisible. Because of this, it is only known to exist via its gravitational effects on the visible Universe (see e.g. heic1215a<\/a>).<\/p>\n To learn more about this mysterious substance, researchers can study it in a way similar to experiments on visible matter, by watching what happens when it bumps into things. On Earth, scientists use particle accelerators<\/a> to find out more about the properties of different particles.<\/p>\n Physicists can investigate what substances are made of by accelerating particles into a collision, and examining the properties and trajectory of the resulting debris.<\/p>\n For this reason, researchers look at vast collections of galaxies, called galaxy clusters<\/a>, where collisions involving dark matter happen naturally and where it exists in vast enough quantities to see the effects of collisions.<\/p>\n Clusters of galaxies are a swarm of galaxies permeated by a sea of hot X-ray emitting ionised hydrogen gas that is all embedded in a massive cloud of dark matter. It is the interactions of these, the most massive structures in the Universe that are observed to test dark matter’s properties.<\/p>\n A spectacular head-on collision between two galaxies fueled the unusual triangular-shaped star-birthing frenzy, as captured in a new image from NASA\u2019s Hubble Space Telescope. Galaxies are made of three main ingredients: stars, clouds of gas and dark matter.<\/p>\n During collisions, the clouds of gas spread throughout the galaxies crash into each other and slow down or stop. The gas-gas interaction in cluster collisions is very strong, while the gas-star drag is weak. In a similar way to a soap bubble and a bullet in the wind where the bubble would interact a great deal more with the wind than the bullet.<\/p>\n The stars are much less affected by the drag from the gas and, because of the huge gaps between them, do not have a slowing effect on each other, though if two stars did collide the frictional forces would be huge.<\/p>\n Said lead author David Harvey of the \u00c9cole Polytechnique F\u00e9d\u00e9rale de Lausanne<\/a> (EPFL) in Switzerland:<\/p>\n “We know how gas and stars react to these cosmic crashes and where they emerge from the wreckage. Comparing how dark matter behaves can help us to narrow down what it actually is.”<\/p><\/blockquote>\n Harvey and his team used data from the NASA\/ESA Hubble Space Telescope and NASA’s Chandra X-ray Observatory to study 72 large cluster collisions. The collisions happened at different times, and are seen from different angles; some from the side, and others head-on.<\/p>\n To find out where the dark matter was located in the cluster the researchers studied the light from galaxies behind the cluster whose light had been magnified and distorted by the mass in the cluster. Because they have a good idea of the visible mass in the cluster, the amount the light is distorted tells them how much dark matter there is in a region.<\/p>\n The team found that, like the stars, the dark matter continued straight through the violent collisions without slowing down. However, unlike in the case of the stars, this is not because the dark matter is far away from other dark matter during the collisions.<\/p>\n The leading theory is that dark matter is spread evenly throughout the galaxy clusters so dark matter particles<\/a> frequently get very close to each other. The reason the dark matter doesn’t slow down is because not only does it not interact with visible particles, it also interacts even less with other dark matter than previously thought.<\/p>\n “A previous study had seen similar behaviour in the Bullet Cluster,” says team member Richard Massey of Durham University, UK. “But it’s difficult to interpret what you’re seeing if you have just one example. Each collision takes hundreds of millions of years, so in a human lifetime we only get to see one freeze-frame from a single camera angle. Now that we have studied so many more collisions, we can start to piece together the full movie and better understand what is going on.”<\/p><\/blockquote>\n By finding that dark matter interacts with itself even less than previously thought, the team have successfully narrowed down the properties of dark matter. Particle physics theorists have to keep looking, but they now have a smaller set of unknowns to work with when building their models.<\/p>\nGalaxy Cluster Interactions<\/h2>\n
Galactic Collisions<\/h2>\n
![\"galaxy](\"https:\/\/techfragments.com\/wp-content\/uploads\/2015\/03\/galaxy-collision-space-triangle-hubble-nasa.jpg\")
Credit: NASA, ESA, Julianne Dalcanton (Center for Computational Astrophysics \/ Flatiron Inst. and University of Washington); Image Processing: Joseph DePasquale (STScI)<\/p><\/div>\nThe Supersymmetry Model<\/h2>\n