No one knows for certain. Theoreticians believe it is at its heart a 'quantum field' just like the fields associated with the other 3 fundamental forces. It would then be a shimmering cloud of gravitons which come and go according to the dictates of Heisenberg's Uncertainty Principle. The graviton particles carry a 'spin' of 2 units, have no rest mass, and travel as packets of space-time curvature, at the speed of light.