These galaxy clusters are part of a large study using Chandra and Hubble that sets new limits on how dark matter - the mysterious substance that makes up most of the matter in the Universe - interacts with itself. The hot gas that envelopes the clusters glows brightly in X-rays detected by Chandra (pink). When combined with Hubble's visible light data, astronomers can map where the stars and hot gas are after the collision, as well as the inferred distribution of dark matter (blue) through the effect of gravitational lensing.