Gravitational Instability of the Universe
Even when the missing mass is found for the galaxies, it will not explain why the universe does not collapse, because the universe as a whole is not rotating. One can observe that the spin axes of different galaxies point in different directions. On average there is no net spin. If the galaxies are not revolving around a common center, what keeps gravity from pulling them all together into a solid mass? Einstein did not see any way that the universe could exist forever in its present, extended state. He looked over his equations to see if he could patch them up.