That's a bad analogy. Mindbound already addressed black holes. Dark matter is however something where we know there's something weird going on, and the simplest explanation that doesn't involve completely throwing out general relativity is to posit that there's mass out there we can't see. Specific variants of that hypothesis each make different testable predictions (indeed we can even just test the general dark matter hypothesis by looking for other signs of dark matter, such as through gravitational lensing which confirms the presence of dark matter). The comparison doesn't hold.