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Galaxy clusters collide—dark matter still a mystery

 

....The researchers analyzed the collision data to measure the change in momentum of dark matter when two galaxy clusters crashed into each other. Experiments on Earth, e.g. in the Large Hadron Collider, show us that when subatomic particles interact, they exchange momentum. Therefore, depending on what happened to the dark matter after the collision, the researchers could draw conclusions about its nature.

To test the theory that dark matter consists of particles, the study worked with two possible scenarios: Either the particles of the dark matter interacted frequently but exchanged little momentum, or they interacted rarely but exchanged a lot of momentum. In the first case, dark matter would slow down after the collision, because the frequent particle interactions would cause an additional "drag". In the second scenario, dark matter would tend to be scattered away and lost into space.

Surprisingly, the study discovered that dark matters in galaxy cluster collisions simply pass through each other. This implies that dark matter particles do not interact with themselves, which would have caused dark matter to slow down. Instead, it appears that while dark matter could interact "non-gravitationally" with visible matter, this is not the case when it interacts with itself.

More importantly, the study challenges the view that dark matter consists of proton-like particles - or perhaps any particles whatsoever. "We have now pushed the probability of two 'dark matter particles' interacting below the probability of two actual protons interacting, which means that dark matter is unlikely to consist of just 'dark-protons'," says David Harvey. "If it did, we would expect to see them 'bounce' off each other".

Read more at: http://phys.org/news/2015-03-galaxy-clusters-collidedark-mystery.html#jCp