Two billion light-years away, galaxy cluster Abell 1689 is one of the most massive objects in the Universe. In this view from the Hubble Space Telescope’s Advanced Camera for Surveys, Abell 1689 is seen to warp space as predicted by Einstein’s theory of gravity — bending light from individual galaxies which lie behind the cluster to produce multiple, curved images. Almost every fuzzy yellow patch in the above image is an entire galaxy. A close inspection, however, shows that many background galaxies are strangely magnified and distorted into long curving arcs (they are not scratches on the lens of the telescope) by the gravitational lens deflections of the cluster. The power of this enormous gravitational lens depends on its mass, but the visible matter only accounts for about one percent of the mass needed to make the arcing images of background galaxies. Computer analyses of the placement and smoothness of these arcs indicate that in addition to the matter in the galaxies you can see, the cluster must also contain a significant amount of dark matter such as the model digitally superposed in purple. In fact, most of the gravitational mass required to warp space enough to explain this cosmic scale lensing is in the form of still mysterious dark matter. As the dominant source of the cluster’s gravity, the dark matter’s unseen presence is mapped out by the lensed arcs and distorted background galaxy images.
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