An extremely rich cluster of galaxies two billion light years away, Abell 1689 is one of the most massive "objects" in the known Universe. The huge mass of the cluster so thoroughly warps the fabric of space-time in its vicinity that here and there, circular arcs can be seen, which are images of more distant galaxies, distorted by the passage of their light through the gravitational field of the cluster. (In the smaller scale image at Clusters of Galaxies, it is hard to see the faint arcs scattered all around the cluster; that is the reason for the much larger image shown here.) However, it is not the mass of the visible galaxies, immense though it is, that bends the fabric of space in this way. Based on the strength of the "lens" created by the cluster, the mass of the cluster must be a hundred times greater than that of the visible stars. This result, extreme though it seems, is fairly typical for galaxies and clusters of galaxies. Every study of the mass of galaxies and clusters of galaxies yields estimates of the gravitational force that are ten to twenty times greater than the estimated mass of the visible stars (see Dark Matter In Galaxies for an introduction to this topic). The image below uses computer calculations of the lensing power of the cluster (based on the arcs and distortions of the galaxies seen all around it) to show the distribution and amount of dark matter in the cluster. That distribution, shown in blue, is most concentrated where the galaxies are concentrated. This is an almost universal result. Wherever there is more visible matter, there is more dark matter. Save for very unusual circumstances, the two always go together. Why that is, is completely unknown. Someday, we will know the answer; and once we do, it will probably be (or at least seem) so simple and obvious that we will wonder why we didn't realize it in the first place (save for the fact that in the absence of any way of observing dark matter, it is far easier to speculate about it than to solve the problem, and speculation tends to muddy the waters which good data clarify). (Image credit Hubblesite and NASA, ESA, E. Jullo (Jet Propulsion Laboratory), P. Natarajan (Yale University), and J.-P. Kneib (Laboratoire d'Astrophysique de Marseille, CNRS, France), Acknowledgment: H. Ford and N. Benitez (Johns Hopkins University), and T. Broadhurst (Tel Aviv University))