Well I’m not the one to argue with you but dark matter is only a thing because our current assumptions are a thing.
If we had to change our thinking because of new knowledge of some fundamental assumption (such as the reason for red shift), it could very well do away with dark matter. I’m sure such a change of thinking will seem as ridiculous to scientists today as heliocentrism seemed to astronomers of Galileo’s day.
Yes, there may be an alternative answer. Just need to throw general relativity out of the window.
To be fair, future physics may indeed throw it away so there’s that.
It is just not one observation that the new assumption has to fit. There are multiple options that fit some but not others, dark matter fits most we just don’t know what it’s made up of.
