Galaxy clusters are the largest stable structures in our Universe. Seven billion light years away and with two million billion times the mass of our Sun, a cluster nicknamed “El Gordo” – “the Fat One” in Spanish – has been spotted and is said to be currently undergoing a merger and growing even larger.

Galaxy clusters yield many cosmic superlatives; mergers such as the one that El Gordo appears to be undergoing are the most energetic events in the Universe, as vast amounts of matter – and the mysterious dark matter – crash into each other at breakneck speeds. Mapping out the process of cluster growth will be critical to understand the interplay between the forces of dark matter and dark energy.