This finding narrows down the options for what this mysterious substance might be.

"Dark matter is an enigma we have long sought to unravel. With the combined capabilities of these great observatories, we are ever closer to understanding this cosmic phenomenon," said John Grunsfeld, assistant administrator of the NASA's science mission directorate in Washington, DC.Dark matter is an invisible matter that makes up most of the mass of the universe.

To learn more about dark matter and test such theories, researchers study it in a way similar to experiments on visible matter -- by watching what happens when it bumps into other objects. In this case, the colliding objects under observation are galaxy clusters.

Galaxy clusters are made of three main ingredients: galaxies, gas clouds, and dark matter. During collisions, the gas clouds surrounding galaxies crash into each other and slow down or stop.

The galaxies are much less affected by the drag from the gas and, because of the huge gaps between the stars within them, do not slow each other down.

"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 the study's lead author David Harvey of the Ecole Polytechnique Federale de Lausanne (EPFL) in Switzerland.

For the study, Harvey and his team studied 72 large cluster collisions.The collisions happened at different times and were viewed from different angles -- some from the side, and others head-on. The team found that, like the galaxies, the dark matter continued straight through the violent collisions without slowing down much.

This means dark matter does not interact with visible particles and flies by other dark matter with much less interaction than previously thought. Had the dark matter dragged against other dark matter, the distribution of galaxies would have shifted.

With this discovery, the team has successfully narrowed down the properties of dark matter.Dark matter may have rich and complex properties, and there are still several other types of interactions to study.

These latest results rule out interactions that create a strong frictional force, causing dark matter to slow down during collisions. The team also is looking to study collisions involving individual galaxies, which are much more common.


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