We are told that as the Universe expands, the 'temp' of it cools and we now have the microwave background radiation as a remnant. Fine. Considering the energy density of the universe, energy is indeed conserved as it expands and cools. However, what about an individual photon, who's energy is given by E=hf. Its wavelength has increased due to the expansion of spacetime, reducing its frequency, and hence its energy. Where did its energy go?