Transcript: In the standard model of cosmology structure formation occurs in a top down way which means that the smallest objects, galaxies, form first and then subsequently cluster to form clusters of galaxies and eventually superclusters of galaxies. The largest structures therefore should be the youngest, and indeed the local supercluster of galaxies in the nearby universe is only just forming. Clusters in the local universe are observed to be relaxed, that is symmetric, and apparently gravitationally stable, the galaxies having had a number of orbits in and out of the cluster. But by high redshift, the time available for forming large structures is much less so astronomers anticipate that rich clusters should be very rare at high redshift. The density or number of clusters at high redshift is thus a test of the standard cosmological model, and astronomers put a large effort into trying to find high redshift clusters. It’s very difficult to find clusters beyond a redshift of one which means that mass concentrations that large were very rare in the first few billion years of the universe.