Galaxy clusters are made of three main ingredients: galaxies, clouds of gas, and dark matter. During collisions, the clouds of gas enveloping the 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, they do not have a slowing effect on each other.
“We know how gas and galaxies react to these cosmic crashes and where they emerge from the wreckage. Comparing how dark matter behaves can help us narrow down what it actually is,” said David Harvey of the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland