So, IIRC, the big deal about dark energy is that the universe appears to be expanding. How do we know the universe is expanding? As far as I know (and that's not much), we know because of redshifting of light on the spectrum as it moves away, as per the Doppler effect, which has been observed in distant galaxies and stars. If I'm not mistaken, then, the single bit of evidence we have for the expansion of the universe, is that one physical phenomenon. But, what if light redshifts as it travels? Would naturally redshifting light as it travels through a vacuum violate any known laws or observations, and would it explain the mysterious apparent acceleration of the expansion of the universe?