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Scientists learn more about dark matter when stars collide

Dark matter – how does it work? That’s something that astronomers are still trying to figure out, but new research from École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, the University of Edinburgh and Durham University have found one way it doesn’t work. When stars collide, dark matter doesn’t interact with itself, or anything – it breezes through the collision versus spreading in a fluid manner, like other matter.

“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,” explained Dr David Harvey of the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, lead author of the study.

The method for observing dark matter, an invisible particle, is as scientific as you’d imagine – the scientists watch as stuff bumps into other stuff. Of course, researchers need events on a cosmic scale, so they looked to galaxy clusters, vast collections of galaxies where dark matter collisions happen naturally.

When two stars collide, most of the accompanying matter – gasses, dust, etc., splatters in a fluid fashion. The stars, however, pass through one another. Dark matter does the same thing, not interacting with itself and carrying on as though nothing happened. While this doesn’t give researchers much evidence of what dark matter is, they can begin to rule out what it isn’t.

It’s not because dark matter is rare in the universe, and therefore far away from itself, either. Quite the contrary, in fact. One theory is that dark matter is so common in the universe and evenly spaced, it doesn’t have much room in which to react. Of course, since it’s invisible, it’s very difficult for scientists to work with. An earlier study of the Bullet Cluster saw dark matter behave similarly (that is, not at all).

“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,” said Dr Richard Massey, in the Institute for Computational Cosmology, Durham University.

Star collisions aren’t the only way to observe dark matter. What scientists do know from this study is that they can rule out interactions that create a strong frictional force, causing dark matter to slow down during collisions. It’s possible, in fact, that other interactions could cause dark matter to bounce off itself, casting it away from collisions. Or, it could exist in blob-like formations, simply changing shape when objects collide. The scientists really don’t know.

“There are still several viable candidates for dark matter, so the game is not over, but we are getting nearer to an answer,” Dr Harvey said. “These ‘Astronomically Large’ particle colliders are finally letting us glimpse the dark world all around us, but just out of reach.”

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