It is difficult to hide a galaxy behind a cluster of galaxies. The clusters gravity will act on the light from the distant galaxy like a huge lens, pulling the light around the sides of the cluster and greatly distorting it. The cluster CL2244-02 is composed of many yellow galaxies and is lensing the image of the bluish-white background galaxy into a huge arc. Another lensed galaxy can faintly be seen, just above the arc, in red.

Such a smooth arc can only be created if most of the foreground clusters material is smoothly distributed dark matter (a little understood form of matter) and therefore not concentrated in the yellow galaxies visible. Analysing these gravitational arcs gives astronomers a method to estimate the dark matter distribution in clusters of galaxies.

Photo Credit: ISAAC, 8.2 metre VLT, ESO