I have a background in physics (but not astrophysics) and to me MOND seems much more ad hoc than dark matter does. Dark matter posits that there is some kind of particle we have not observed directly that has a large effect on cosmological scales. We already believe that the standard model of particle physics is incomplete, so this is at least plausible. These particles are not in anyway "unobservable" in an absolute sense, they must at least interact via gravity in order to do what we need them to. They may even interact via the nuclear forces. They just don't interact via the electromagnetic force. We can make hypotheses about these particles, put constraints on their properties from observational data, use particle physics theory to come up with candidates, and do experiments to find them. They are not "unobservable", the are simply hard to observe and require advances in experimental technology. MOND on the other hand was discounted in the early days exactly because it was a fudge factor in Newton's second law. We saw some anomaly in galaxy rotation curves, MOND fixed it by adding a fudge factor to Newton, a theory which we already knew at the time to be incomplete! That is why most physicists preferred dark matter to MOND. Since then, other observational data has increased the evidence for the existence of dark matter (most famously the Bullet Cluster mentioned by others), making MOND that much more unpopular. There has been some work, like this RelMOND theory, that work to integrate the original MOND theory with general relativity. Critically, however, the way in which RelMOND is doing this is by positing the existence of an additional field that can act in the "clumpy" way required by observation. Basically, they have just reinvented dark matter. So I do not doubt this new theory fits the observation, but it does so by adding a new degree of freedom which plays the part of dark matter. This talk (https://www.youtube.com/watch?v=iu7LDGhSi1A) is somewhat technical, but it goes in to some detail as to why we can't explain current observation by messing around with gravity.