The density of radiant energy given off by stars and luminous matter in the universe is computed and is found to be 6.91\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ erg/sq. cm/sec. as received by the earth. In interstellar space, ${10}^{6}$ light years from the nearest spiral nebula, the energy density will be between 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$ and 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ erg, the former value representing energy from all known nebulae and the latter an extrapolation to the 25th magnitude, further extrapolation being forbidden by red-shift considerations. The latter figures are considerably smaller than the cosmic-ray energy densities found by Millikan and others.