I have chosen to honor PF by publishing my conjecture first here . The Marcus Conjecture, which I formulated a few days ago on one of the cosmological redshift threads, is that the energy lost from the CMB by expansion has gone into the form of dark energy. I shall now show plausibility by calculating the amount of CMB energy in a given volume of space. The amount which the photons in that volume have already lost we know to be about 1100 times their current energy. It will turn out that the amount lost by the photons in the volume is within an order of magnitude of the dark energy in that volume.