How can a massless particle such a photon be the result of electron-positron annihilation? What about the law of conservation of energy? Is a valid explanation that the pair's energy transforms itself into the kinetic energy of the photon, not its mass?
The annihilation process is $e^-+e^+\to 2\gamma$. In the rest frame of the system, the spatial momenta of the particles before and after the event are opposite to each other, but their kinetic energies (the 0-component of the relativistic 4-momentum) add up and all four of them are equal.