Background fluxes (Hz-1) for the various EM bands. This effectively gives the relative photon number densities. Note that the CMB photon density dominates (about 400 cm-3)
Background fluxes plotted as the produce of frequency and flux. This shows the relative energy per decade in frequency. Note that even though the CMB photons are each of low energy, nevertheless, they still dominate the photon energy density of the universe.
Figure from Longair (?) [see also RT 4.1 Raine & Thomas]