Until we get gravity licked there will always be speculation about just what it is and how it's caused. I am wondering if it could be related to the strong nuclear force that holds atoms together. Could it be that this force also has the effect of producing gravity?
Also if this is in any way connected could an external energy source acting on the strong nuclear force change it so that it is no longer attracted to other gravity producing sources of mass, in the same way that changing the polarity of magnet would stop it being attracted to another magnet?
Then we come to anti matter, would anti matter produce gravity just like mass?, would it produce a type of anti gravity or repelling force?, or would it produce a new type of gravity that only affects anti matter?