When light makes a long distance journey it's frequency shifts toward longer wavelengths.
With $E = hc/\lambda$ being the amount of energy in any given photon (see photon energy); as it travels trough space it wavelength is lengthened which means it's loosing energy as it travels through empty space.
As energy is conserved is it being converted to make more space or is there an interaction with vacuum energy that explains what happens to the energy of the photon?
