Right. A lot of "modified gravity" theories have been proposed in recent decades, investigated, and compared to observations; with work they can be made to match up. Dark-matter theories on the other hand do a somewhat better job of matching observations, which is why they have become the best-liked explanation for the galactic rotation curve puzzle.

Scientists didn't just jump to dark matter at the start, and no cosmologist will tell you that dark matter is definitely the explanation — it's simply the explanation that currently seems most likely.