Dark matter is only the "caboose" on a whole train of answer.
When one object totally dominates the gravitational field of some interacting bodies, like the Sun does in the Solar System since it has 99% of the mass, all the objects orbiting the big one can be reasonably approximated as interacting with the central body. This is called the Two-body problem. In the two-body problem, both the angular and linear velocities decrease for objects in circular orbits as the orbital radius increases. In other words, Venus' year is longer than Mercury's, Earth's is longer than Venus', etc. Also, [CORRECTION:] Mercury literally flies through space at greater miles per hour than Venus, etc.
Now, a galaxy's central black hole is the most massive single object in the galaxy, but it is typically only a tiny fraction of the total mass. The mass of a galaxy is dominated by the overall soup of matter reaching out from the center. Since this is not the two-body problem, the Solar System results need not apply. To solve for how objects should behave in a soup, you basically consider just the amount of soup between your given object and the center. Therefore, objects farther out are basically behaving as if they are orbiting a more massive body than objects closer in.
The net result of that is that objects orbiting in a soup should have their angular and linear velocities decrease less than the traditional two-body problem. In fact, we observe that their linear velocities seem to be almost perfectly the same. That means there must be a lot of matter in the soup...
However, we can't find enough light-emitting, ordinary matter to account for how much matter we just inferred was there. This is the first and only part of the story where we need to invoke dark matter.
MOND is a theory that attempts to resolve this final discrepancy in a different way. Instead of inferring lots of mysterious matter of unknown properties, it modifies the math surrounding very slow gravitational interactions (technically, those involving very low accelerations). This math works extremely well, but it hasn't achieved widespread acceptance because there is no "story" or explanation backing up the math at all, and MOND is difficult or impossible to mesh with other, established theories like Special and General Relativity.