Figure 14-28 RIVUXG Isolated Dark Matter in a Cluster of Galaxies (a) This visible-light image of the galaxy cluster 1E0657-56 shows more than a thousand galaxies. The superimposed image in red shows the distribution of the cluster’s hot, X-ray-emitting gas, and the blue image shows the distribution of dark matter as determined by gravitational lensing (see Figure 14-27). (b) We can understand the separation of dark matter and gas in this cluster if we assume that dark matter does not feel any force of fluid resistance. This is what we would expect if dark matter were to respond to gravitational forces only.