It’s more than that. There’s something that doesn’t add up, but if we assume the answer is “dark matter”, we can make predictions about it, that can guide us toward proving or disproving. Similarly, if we assume it’s one of the other theories, we can make predictions on what it must be like.
Dark matter is most straightforward because we understand best how matter acts. How much more matter do we need for the observations to make sense, based on current understanding? Ok, what could that matter be that acts gravitationally but we can’t see? How can we detect that?