Since my view on light is as it is, that light can't lose energy in that way. Becasue of the fact, to me, that light only travels in zero time hence no loss of energy in your example. It can deviate time back and forth, but this effect always cancels out in the big picture.
And for Dark matter we are already getting in elementary particles like the neutrino for example. That before was tought as a "sister" of the Photons and as being massless. But later research oscilations in neutrinos which indicate a small mass. From what i have read neutrinos might well be filling up alot of space, altough this is hard to measure with a particle that has a very low interaction with "common" matter.
Another candidate and one i find very likely and heavy is the Black holes, i belive we have experimentel evidence(do we?, hard to keep track here since everbody writes about them as a matter of fact). These bastards might well constitute a big bunch of all the missing heavy weight champs out there!
Gravitons are one two, altough here i know extremly little about(if these even exists).