I think your response makes sense (neutrinos are another example where the theory predicted an as yet undetected particle and it was eventually found).

But the other reply to your comment is good too - at what point, after failing to detect any OTHER signs of these invisible elephants, do you wonder if there’s other explanations? What it would take to falsify dark matter, given that you can invent it in whatever distribution you like to fit the data?

There’s nothing that logically compels us to assume the existing model of gravity is correct, and its discrepancy with observation is explained by assuming the inputs aren’t what we can observe. It is also logical to consider that the existing model of gravity isn’t quite right. Of course any tweaks to gravity have to be compatible with a lot of other observations but it would be wrong to rule out the possibility of an alternate model.