The Spiderweb protocluster

This image shows the protocluster around the Spiderweb galaxy (formally known as MRC 1138-262). The light that we see in the image shows galaxies at a time when the Universe was only 3 billion years old. Most of the mass in the protocluster does not reside in the galaxies, but in the gas known as the intracluster medium. Because of the mass in the gas, the protocluster is in the process of becoming a massive cluster held together by its own gravity.
Credit: ESO/H. Ford