If dark matter really existed, we would have a prediction of its existence based on the laws of physics. Since there is no theory of dark matter, why pretend it exists?
The laws of physics are our mathematical guesses at how the universe works. They are not fixed since we do not know exactly how the universe works.
Why? For the usual reason one posits an unobserved entity in science—to explain things one observes which the observed entities and laws do not explain.
Personally, I suspect once one gets the quantum theory of gravity right (no, string theory isn’t it) the “need” for ‘dark matter’ and ‘dark energy’ go away.