If the "60-Second Egghead" video that Scientific American has posted explaining 'Dark Energy' is to be believed, it (DE) is responsible for maintaining the separation of the galaxies. Without it, everything would condense; attracted by mutual gravity and the cosmic waltz would not exist. So DE must have some form of 'substance' relative to gravity - sort of like pelaspan in your UPS shipment, it keeps things from bumping into one another.
If we can show that light is affected by gravity as we have seen in various examples of gravitational lenses formed by quasars, could it be that DE also affects light? If so could the Doppler Shift be a result of DE exerting a 'drag' on light passing through it, thereby shifting its frequency towards the red end of the spectrum?