Thought experiment: Assume two galaxies in a galaxy group, initially at rest (with respect to one another). The distance between the centers of the galaxies is r = 1 Mpc. The total mass of each galaxy is mg = 6 ∙ 1042 kg (including dark matter). This is ≈ 3 ∙ 1012 solar masses. The gravitational pull between the galaxies will be F= G ∙ mg2 /r2 = 2.67 ∙ 1030 N, and the corresponding acceleration will be a = F/mg = 4.45 ∙ 10-13 m/s only. (Using G= 6.67 ∙ 10-11 m3kg-1s-2). How can this tiny force withstand the expansion of space (which at this distance is ≈ 70 km/s)?