Newton taught us that bodies with mass attract each other according to the universal law of gravitation (mass-mass attraction) and Einstein taught us that mass and energy are equivalent though his famous formula :E = mc^2. Also, experiments showed us that light (photons, which have no mass (only energy) are influenced by gravity as they have their trajectories warped (mass-energy attraction). But does energy somehow attract energy? Does it somehow clump up in a form other than normal matter?
I was reading a book on General Relativity and wondered about the possibility of this idea.