You mean Neptune, of course.
One more planet among several was hardly a stretch. A prediction was made, observations were performed, and the planet was found. Just as it should be.
Dark matter is in a weaker position than Neptune ever was, given that its nature is unknown, but ultimately it will succeed or fail based on empirical measurements.
I’m not going to go full Popper and say that falsifiability is the end-all-be-all of all knowledge, but statements about the nature of reality need to be tested somehow, and not just through self-consistency. And unlike the perturbation of the orbits or galactic rotation curves, it’s not obvious that there’s a problem that needs to be solved.