We put ball A and ball B by distance R, every atom in ball A has some positive charges and some negative charges, same as atoms in ball B. Now suppose every positive charge in ball A attracts every negative charge and repells every positive charge in ball B, and verse visa. Now we have all the single force add up, we find it equals to f=gxm1xm2/rxr I once did some calculation, it sounds looked right, but now I forgot the details. So, I believe, gravity is the shadow of electrostatic force. An easier way to see this is to put two atoms apart and calculate the forces between all positive and negative charges. You may find I was right. If not, please let me know why, be appreciate.