 With this new understanding about the possibility and impact of dark matter, astronomers turn their attention back to galaxy clusters like the one studied by Zwicky in 1936. Back then, Zwicky suggested that gravitational lensing could be used to better understand dark matter. Our case in point galaxy is known as the bullet cluster. The varial motion of its galaxies indicates that a collision has occurred. Two massive clusters have passed through each other millions of years ago and member galaxies are now flying apart. If we zoom in a bit closer, we can see the telltale arcs of more distant galaxies lensed by the gravity of the bullet cluster. Counting the lensed objects and the estimated amount of light bending involved for each one, a map of the area containing most of the mass of the cluster can be superimposed. We have used blue to indicate the locations where the vast majority of the matter must be located in order to get the observed lensing. Here we have the cluster's hot X-ray emitting gas detected by the Chandra X-ray Observatory. The two pink clumps contain most of the normal matter, sometimes referred to as baryonic matter or matter made up of protons and neutrons. The bullet-shaped clump on the right is the hot gas from one cluster which passed through the hot gas from the other cluster during the collision. When we superimpose the dark, baryonic and visible components of the cluster's mass, we get the full picture. The galaxies and the dark matter have traveled a great deal further than the gas. This indicates that the galaxies and dark matter in the two colliding clusters did not interfere with each other. In other words, they passed through each other without slowing down. On the other hand, during the collision, the gas clouds were slowed by a drag force similar to air resistance. This combination had the effect of separating the gas from the dark matter. This separation is considered to be direct evidence that dark matter exists. Measurements indicate that the galaxy clusters on average have 85% dark matter, 14% intergalactic gas and only 1% stars.