Fritz Zwicky observed that the velocity dispersion of galaxies in the Coma galaxy-cluster appeared to be much too high to keep the galaxies together in a cluster [1]. This conclusion was derived from the virial theorem [2] by assuming that the interstellar gravitational force can be derived from a potential. At the time it was not recognized that this is an unjustified assumption, a fact that is still not properly appreciated nowadays (see [7]). In this note we show that the Coma cluster actually stays together because pairs of stars in an inter-galactic arrangement attract each other essentially by a force decaying according to a 1/r law, where r is the distance of the pair. This type of behavior has originally been proposed by Milgrom [3] to account for galaxy rotation curves. For such a law there are no free bodies at all! Actually, there is already a crossover to this law within a single galaxy. Here, we show that a mechanism of counter-flowing massless gravitons and anti-gravitons leads to such a behavior, namely when this interstellar mechanism starts to obtain competition from the acceleration processes provided by the direction-unspecific cosmic gravitational background radiation.