I am a newbie to cosmology, and have just finished reading "The first three minutes" by Weinberg. On page 76 he relates that in the early universe almost all of the energy was in radiation (photons), and relatively little in matter (the masses of the nuclear particles). But now most of the energy is in matter. I assume the amount of energy in matter remained fairly constant. Is this correct, and if so, again assuming energy is conserved, where did all of the energy in the early radiation go to?