It's far more than a simple fudge factor. It says that there is matter with mass, affecting other matter via gravity, but no other interactions seem possible wi

It's far more than a simple fudge factor. It says that there is matter with mass, affecting other matter via gravity, but no other interactions seem possible with it. That is a sound, almost complete theory with some unknowns around why it doesn't otherwise interact, what it actually is. Dark matter also explains certain gravitational lensing phenomenon, aspects of the CMB, from mass location during galactic collisions (and more). Other theories can possibly explain one or some of those observations, but nothing does a good job (or a simple one) of explaining them all. Dark matter is real enough to make verifiable predictions in other places. Other theories don't.