Researchers using the NASA/ESA Hubble Space Telescope and NASA’s Chandra X-ray Observatory examined galactic collisions to try to determine how dark matter behaves during such events.

Most of the matter in the universe is ‘dark matter’ however, it does not reflect, absorb or radiate light. Astronomers know that it is there because of the gravitational influence it has on other objects.

If you look out the window and see the trees sway and leaves flying around you know it is windy, even if you can’t see or hear the wind. Astronomers know that the dark matter is there because of its influence on objects but scientists know what wind is and where it comes from, the same cannot be said for dark matter.

No one can see it, no one can produce a sample and no one is even quite sure where it comes from. That is one of the questions that researchers with CERN’s Large Hadron Collider are trying to answer. As long as this is the case, there are many questions about the universe that cannot be accurately answered.

So, as detailed in a paper published March 27 in the journal Science, researchers looked at galactic collisions to see how dark matter reacted when it ran into large clusters of other dark matter. The researchers combed through large collections of galaxy clusters, which are large collections of galaxies, where dark matter collisions are frequent.

Researchers know that galaxy are made of stars (and their planets), clouds of gas and dark matter. When two galaxies collide the clouds of gas slow down or stop when they collide. The stars are so far apart that they rarely collide, or are even affected by one another’s gravity, even when one collection of billions of stars pass through another. Researchers do not know what happens when dark matter collides with dark 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,” explains David Harvey of the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, lead author of the new study, in a statement.

Using data on 72 galactic cluster collisions, from different angles the team found that the dark matter clusters did not seem to react to one another at all. However, unlike the stars it was not that they were too far apart.

According to the leading theory, dark matter is spread evenly throughout galactic clusters, so dark matter particles should get very close during a collision. Researchers know that there is no perceived interaction when dark matter collides with visible matter but there doesn’t seem to be any reaction when it collides with dark matter either.

“A previous study had seen similar behaviour in the Bullet Cluster. 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,” says team member Richard Massey of Durham University, UK.

According to the researchers the lack of interaction is still instructive because it narrows down the possible properties of dark matter. It may not, in other words, have been what they were hoping for but they still know more than they did before.

It is still possible that dark matter is interacting with other dark matter, or even visible matter but not in a way that has been observed to date.

The astronomers plan to continue their work by looking at collisions between individual galaxies, which are much more common and will provide a larger sample of images to study.

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