The above image, a composite of optical data, X-ray data, and a reconstructed mass map, is one of the most famous and informative ones in all of astronomy. Known as the Bullet Cluster, it showcases two galaxy clusters that have recently collided. The individual galaxies present within the clusters, like two guns filled with bird shot fired at one another, passed right through one another, as the odds of a collision were exceedingly low. However, the intergalactic gas within each cluster, largely diffuse and making up the majority of the normal matter, collided and heated up, emitting X-rays that we can see today. But when we used our knowledge of General Relativity and the bending of background light to reconstruct where the mass must be, we found it alongside the galaxies, not with the intra-cluster matter.