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It is difficult to hide a galaxy behind a cluster of galaxies. The closer cluster's gravity will act like a huge lens, pulling images of the distant galaxy around the sides and greatly distorting them. This is just the case observed in the above recently released image from the VLT. The cluster CL2244-02 is composed of many yellow galaxies and is lensing the image of a blue-white background galaxy into a huge arc. Careful inspection of the image will reveal at least one other lensed background galaxy appearing in red. The foreground cluster can only create such a smooth arc if most of its mass is smoothly distributed dark matter - and therefore not concentrated in the yellow galaxies visible. Analyzing these gravitational arcs gives astronomers a method to estimate the dark matter distribution in clusters of galaxies.
