The kinetic energy in the motions of particles around at that time would have been so enormous as to have completely overwhelmed the particles’ comparatively tiny rest energies (E=mc2, for a particle of rest-mass m). Thus, the rest-mass of the particles would have been effectively irrelevant—as good as zero as far as the relevant dynamical processes are concerned. The contents of the universe, at extremely early times, would have consisted of effectively massless particles.
