When astronomers study the rotation of distant galaxies, they immediately come up against a puzzle. The stars are held together by gravity, which prevents them being slung into intergalactic space as the galaxies rotate. Indeed, astronomers can calculate the amount of gravity based on the mass of the stars they can see.
The puzzle is that the outermost parts of these galaxies are moving too quickly. There does not seem to be enough of this mass to stop these stars from flying away.
The fact that this does not happen is one of the great mysteries of modern cosmology. Some force must be holding galaxies together but astronomers do not know where it comes from.
