The simplest explanation of gravity is that of Newton, where gravity is force, not mass. The amount of matter in Milkyway is found to be not enough to hold the galaxy together if we use the Newtonian law of gravity where G is universal. Naturally, it is suspected that there is DM in the galaxy. But a speed dependent gravitational constant (G proportional to square of the speed) removes the need for DM.Gravity is mass. It's the redistribution of time speeds in space proximate to matter.
Whether Newton or Einstein provides the correct explanation of gravity is something that can be resolved only after our theoretical physics becomes complete in all respects. Till then it is an open question.