The theory of the Holographic Universe
The total summation of energy of our universe is about zero. It can also be said that the total mass is zero. Let's compare our universe with a photon of this universe. If we call the our universe A, and call that photon universe B, then through universe A we can see universe B is a particle moving in light speed, and vice versa. So in a photon universe our universe is no more than a photon. In fact, we can imagine all the particles are just like other universes. Our universe includes all the other universes, and other universes also include our universe.