The galaxy collisions that shed light on unseen parallel Universe

Dozens of cosmic collisions between massive clusters of galaxies have shed much-needed light on dark matter – the mysterious substance that makes up most of the Universe, but which is invisible to all telescopes.

Scientists believe they can eliminate some of the more “exotic” theories about dark matter after studying 72 different collisions between galaxy clusters, each of which contain up to a thousand galaxies colliding at a speed of 1,000 kilometres per second over a period of 100 million years.

The results of a study appear to confirm dark matter makes up an unseen, parallel Universe composed of sub-atomic particles that do not interact at all with the visible matter of the stars and planets.

“We haven’t shown it is any one thing, but we’ve ruled out many other things that it can be,” said Richard Massey, an astronomer at Durham University, one of the researchers involved in the study published in the journal Science.

“We have shown that dark matter interacts even less than previously thought. This rules out several proposed types of dark matter particles. There are still several viable candidates, so the game is not over, but we’re getting closer to an answer,” Dr Massey said.

Previous theories have suggested that dark matter is some kind of quantum defect that appeared after the Big Bang birth of the Universe, or that it is a weirdly modified form of gravity. However, the findings suggest that dark matter is another kind of sub-atomic particle, possibly forming a parallel universe of “supersymmetry” filled with supersymmeterical matter that behaves like an invisible mirror-image of ordinary matter.

“We know this stuff exists. I’m trying to measure the properties it has. If supersymmetry exists it may mean every particle of matter has a supersymmetrical mirror image,” Dr Massey said. “It could mean there are lots of different types of dark matter. It adds a lot of flavour and character to dark matter. It makes it more interesting.”

The study was based on observing the collisions hundreds of thousands of light years away, which revealed that dark matter interacts very little, even with itself.

While the Large Hadron Collider at the Cern nuclear laboratory in Geneva measures collisions between sub-atomic particles on Earth, the Hubble Space Telescope and Chandra X-ray Space Observatory can observe much bigger collisions in space, Dr Massey said.

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“They [Cern] smash bits of protons together to see what they are made of, but they can’t smash bits of dark matter together. We can look around the Universe to see where this happens and study how these big particle colliders in space crash dark matter together.”

Although dark matter is invisible, it can be detected and mapped by the gravitational distortions is produces on starlight. These three-dimensional maps have already revealed that it acts like an invisible scaffold around which ordinary matter accumulates.

The astronomers were able to superimpose dark matter maps onto images of 72 galaxy cluster collisions, observed from various directions in space, to see how it behaved during the clashes. This allowed scientists to study the different stages of various collisions to build up a moving image over time.

“Galaxies are made of three ingredients: stars, swirling clouds of gas, and dark matter. During collisions, stars almost always pass straight through each other. They are pinpoints of matter separated by vast empty space,” Dr Massey said. “Conversely, the clouds of gas smash into each other, and stop, like a giant car crash. Dark matter behaves somewhere between these two extremes, and its trajectory out of a collision reveals its properties. Now… we can start to piece together the full movie and better understand.”

Dark matter was not measurably slowed down when the ordinary matter of stars and galaxies collided in a well-known galaxy collision called the Bullet Cluster.

Dr Massey said: “This lack of interaction with our world suggested that dark matter might be one of the hypothesised supersymmetric particles… If the theory were correct, there should be at least one type of supersymmetric partner for all of the different types of particle we currently know.”