You're suggesting either an interaction with something unknown, which I'm extremely skeptical about because there aren't any known interactions of light with particles that would simply shift it's frequency, and those particles would have to be visible because they do interact with light. So an impossible interaction.
Or you're suggesting a violation of conservation of energy and momentum of light over large distances, by vacuum itself. Which would probably be detectable here. Not even sure how you can suggest it in a consistent way.
Then there's the reasonable option, that it's just gravity and the spacetime metric itself changing, which is the leading theory anyways. Explaining it away with some invisible matter that acts only gravitationally, like you suggested, is literally what they are doing.
I'll be up front about this. I'm in way over my head in this discussion, but in my mind, still, not all interactions are with particles. There are fields such as gravity, that affect wavelength.
And what about particles we do see? What if the debris we know is out there (the stuff of collisions and non-congealed leftover particulates, early gases since its many light years away) over immense distances have an accumulative affect? We see the red sunset but do not discern any individual particles with our eyes.
Then there are the things we don't see. Dark matter... and so much of it. I'm guessing that disproving the relevance of all of this has proven tedious, and recounting it to a newbie like myself even more tiresome!
Or you're suggesting a violation of conservation of energy and momentum of light over large distances, by vacuum itself. Which would probably be detectable here. Not even sure how you can suggest it in a consistent way.
Then there's the reasonable option, that it's just gravity and the spacetime metric itself changing, which is the leading theory anyways. Explaining it away with some invisible matter that acts only gravitationally, like you suggested, is literally what they are doing.