Ancient galaxy clusters have been described as “the dark skeletons of the cosmos.” Astronomers at the Instituto de Astrofísica de Canarias (IAC) have found the most densely populated galaxy cluster in formation in the primitive universe. The researchers predict that this structure, among the largest astronomical objects in the Universe, which is at a distance of 12.5 billion light years from us, will have evolved becoming a cluster similar to the 1,300 galaxies of the Virgo Cluster, a neighbor of the Local Group of galaxies to which harbors our home galaxy, the Milky Way. The Dark Skeletons Clusters of galaxies –“dark skeletons”– shaped by the underlying distribution of dark matter” –are groups of galaxies which remain together because of the action of gravity. To understand the evolution of these “cities of galaxies” scientists look for structures in formation, the so-called galaxy protoclusters, in the early universe. A protocluster is a structure of galaxies still in the process of forming a cluster.