Dark matter is not merely an observation. It is a class of theories to explain that observation. Specifically, if your explanation for the underlying observatio

Dark matter is not merely an observation. It is a class of theories to explain that observation. Specifically, if your explanation for the underlying observation is of the form "there is some matter-like stuff with mass that is causing the observed effects through standard G.R. gravity. We haven't otherwised observed it because X", then you have a dark matter theory. Typically X involves week to non-existant interactions through other forces. If your explanation for the observation involves saying "General Relativity is wrong", or positing the modification or addition of some other force, it may account for the observation behind dark matter; it may even be correct; but it is not a dark matter theory. Assuming some dark matter theory is proven correct, we will probably stop calling it dark matter when it is discovered. Although we still call nuclearly bound protons and neutrons with electrically bound electrons an "atom", even though we have since disproven the hypothesis that such objects are atomic. If the answer ends up not being a dark matter theory, we will probably just call it "gravity".