This theory (when appropriately hacked) describes rotation curves of galaxies extremely well. Unfortunately it doesn't work at larger scales. There are many nice videos on YouTube by Sean Carroll on MOND, which explain it's limits, albeit for the fairly educated layperson.
Its possible but there is a lot of indirect evidence for dark matter, it fits observation well even if we cant prove it. When gravity calculations give an answer that conflicts with observation, sometimes the equation for gravity ends up being wrong (orbit of mercury) and sometimes you find missing mass (discovery of neptune). I don't know what more probable means in this context, the experts are at least as smart as you and me, they favor some kind of missing mass.