https://en.wikipedia.org/wiki/Graviton
Some scientists seem to think the graviton is the massless particle that is responsible for gravitation (although undiscovered).
I personally believe that gravity works in a similar manner to electricity, in that in order for their to be current, electrons have to travel in a flow, coming from the source of induction and back again. Meaning, that if a graviton, as we observe is flowing towards mass and pushing matter towards larger matter, then it must be flowing from somewhere. The graviton flows in a circuit between dimensions or universes and as it flows in from them and across space/time, that flow is what expands the galaxies away from each other (dark energy) and is what causes galaxies to behave in the manner that scientists attribute to unseen and undetectable matter (Dark Matter).