text: Galaxies and their dark matter haloes form from small density perturbations in the expanding Universe. These overdense regions expand more slowly than the rest of the Universe and eventually "turn around" – start collapsing instead of expanding. The blue dots in this figure (for a standard LCDM reference model) show that at this turnaround epoch, these galaxy formation regions are positively curved. If the Universe were flat, galaxies could not be formed. Roukema & Ostrowski 2019, JCAP 12, 049 [arXiv:1902.09064], https://codeberg.org/boud/1902.09064