Photons do not "diminish" in energy except via red shifting (where does the energy go? into the expansion of the universe!). Light scatters, gets absorbed and

Photons do not "diminish" in energy except via red shifting (where does the energy go? into the expansion of the universe!). Light scatters, gets absorbed and re-emitted, it's true, but if distances don't change then the universe would still be much brighter than it is today.