Never seen this addressed anywhere, and maybe it doesn't matter; but, regarding the cosmic background radiation, in any given instant, how many 2.73K (on average) photons are in a given volume ? See, we would measure the same background temperature with our instruments over a range of 'densities', but the actual energy content could vary. (or does the temperature finding of 2.73K take that into account ?) And then a follow up, what would the total energy content in the entire observable universe be for the CMB? And then what is the mass equivalent of that ? (solve for M instead of E in E=MC\ 2) I know this hasn't been overlooked in the field of cosmology, it just hasn't filtered down to my level.