The complete question was: Is it true that all matter is simply condensed energy? The conversion between mass and energy is so ubiquitous in physics, that most physicists only know the mass of particles in the context of their equivalent energy. Now you’ve balanced charge, but violated lepton flavor (electrons have “electron flavor 1”). To balance everything you need to add an anti-electron neutrino (electron flavor -1) to the mix. The mean energy of the photons flying about was more than enough to generate new particles.