Using the baryon-photon ratio to calculate the primordial abundances of baryonic matter:

The photon density = 3.7 x 10^{2}/cm^{-3} Baryon density = (Universe mass density)/Mass_Proton = 4.2 x 10^{-31} g cm^{-3} / 1.66 x 10^{-24} g = ~2.5 x 10^{-7} Baryon density / photon density = 2.5 x 10^{-7} / 3.7 x 10^{2} = ~6.7 x 10^{-10} For every baryon there are ~10^{10} photons.