As far as I know in classical mechanics of Newton energy is merely an attribute of matter (more a defined concept rather than a starting point). With Einstein this changes (or at least so the formula E=mc^2 would imply). Energy is now equivalent to matter (no longer is it merely one of its properties). My question is then, in addition to the energy of the electromnagnetic waves (light), what other forms of energy (if any) could exist should e.g. all the matter in the universe transform itself to energy (as far as the modern theories predict). Any thoughts on this subject would be greatly appreciated.