Galaxy clusters are the largest objects in the universe held together by gravity. They typically contain hundreds of galaxies (stars, gas and dust), spread over a region of whose size is roughly 1025 cm. Their total masses exceed 1048 gm. They contain enormous amounts of superheated gas, with temperatures of tens of millions of degrees, which is invisible to optical telescopes, while glows brightly in X-rays, and can be observed across millions of light years between the galaxies. Also, they contain dark matter, a mysterious form of matter that has so far escaped direct detection with any type of telescope, but makes its presence felt through its gravitational pull on the galaxies and hot gas.

Abell 2744, well-known also as Pandora’s Cluster, is a giant galaxy cluster resulting from the simultaneous pile-up of at least four separate, smaller galaxy clusters that took place over a span of 350 million years, and is located approximately 4 billion light years from Earth. The galaxies in the cluster make up less than five percent of its mass. This image of Pandora’s galaxy cluster combines X-rays from Chandra (diffuse blue emission) with optical light data from Hubble (red, green, and blue).