Observations of colliding galaxy clusters have shown that dark matter interacts weakly not only with visible matter but also with itself. This information, found using the Hubble Space Telescope and the Chandra X-ray Observatory, will allow theorists to refine their scientific models to fit the newly discovered properties of dark matter.
Galaxy clusters consist of stars, gas clouds and dark matter. The stars are very widely spaced, so they are unlikely to collide or exert a slowing effect on each other; they are also mostly unaffected by the drag of the gas clouds. Conversely, dark matter is currently believed to be evenly distributed throughout the clusters, meaning that frequent collisions, and hence slowing effects, should be very likely.
Despite this, observations of 72 galaxy cluster collisions, all happening at different times and in different orientations, showed that dark matter passes straight through galaxies just as the stars do. This means that dark matter particles don’t interact as strongly with each other as previously thought, even over short distances. The study expands on previous observations of the Bullet Cluster, but is more conclusive given the availability of a much greater data set.
These results show that dark matter does not interact in a way that involves particles slowing down due to strong frictional forces. However, it does not eliminate the possibility of dark matter experiencing interactions that cause the particles to change shape or bounce off each other, so there is further scope for discovery through investigating these phenomena in the future.