Time is increasing to the right. Radii of dark matter halos are represented by the blue circles, and the size of their core regions is represented by the red dots. Many little halos at early times merge to form larger halos, which subsequently merge into one big halo. Image: Risa H. Wechsler, James S. Bullock, Joel R. Primack, Andrey V. Kravtsov, and Avishai Dekel, “Concentrations of Dark Halos from Their Assembly Histories,”Astrophysical Journal 566 (2002): 52–70, fig. 2.