The Theories of General and Special Relativity show that matter is energy and energy is matter, this is the famous equation e=mc^2. The thing about Einsteins equations is that the physics of them falls apart when you are looking at a small scale system much like how Newtonian physics couldn't properly explain the movement of planetary bodies. Although Relativity did replace Newtonian physics it couldn't account for things on the Quantum level, which is why we have Quantum physics.
