We have a model and weer have observations. The two don't match, so there is a problem with at least one (model or observations). MOND posits the problem is with the model and proposes refined models. These are refutable and indeed, counterevidence is often found.

Dark matter posits the problem is worth observations. Its solution is to propose a tiny particle that is basically invisible. For every situation, you then can invent an amount and a distribution of these tiny, invisible, undetectable particles to match observations and thereby substantiate your theory.

It's not really surprising that so far, folks have succeeded in inventing a distribution of undetectable clouds of particles, clouds weighing in at multiples of the solar system's combined mass.