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.
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.