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 .
Media Credits & Attribution
Credit: ISAAC, 8.2-meter VLT, ESO