"If you take a galaxy and try to make it bigger, it becomes a cluster of galaxies, not a galaxy. If you try to make it smaller than that, it seems to blow itself apart." -Jeremiah P. Ostriker
13.8 billion years ago, the Universe as we know it was born with no stars, no clusters and no galaxies. But over time, gravitation has built up all sorts of complex structures, with the largest galaxy cluster today, El Gordo, weighing in at 3 quadrillion Suns.
But back when the Universe was just a quarter of its present age, the cluster IDCS J1426.5+3508 already has a mass of 500 trillion Suns, a mass that's been measured by three different methods. By time we fast-forward to today, this cluster is probably the most massive one contained within our visible Universe.