There's been some recent theories that mass creates a condensed field of gravitons, and these are a sort of linking halfway-point between 9 of the 10 dimensions. The theory also postulates that this "dilution" between all dimensions, as opposed to other forces like electro magnetism which are in our realm, is why gravity is the weakest of the 4 universal forces.


Nothing to do with why mass creates gravity, I know, but I found it to be one of Brian Greene's more interesting tidbits (not that they aren't all interesting).