We have a theory and observations. Observations don't match theory. So, at least one of them is incomplete.
Dark matter puts the emphasis on observations(that is: assume the overall theory is roughly correct): there are things out there that, if we would have observed them, would make the theory fit.
Alternatives for gravity focus on the theory (accepting observations as roughly complete): if we change the theory, it should fit the observations.
TL;DR we ought to have A=B, but there's a mismatch. The problem could be due to either A or B. Or both.
Both should be worked on - at least until one can be fully ruled out.
Dark matter puts the emphasis on observations(that is: assume the overall theory is roughly correct): there are things out there that, if we would have observed them, would make the theory fit.
Alternatives for gravity focus on the theory (accepting observations as roughly complete): if we change the theory, it should fit the observations.
TL;DR we ought to have A=B, but there's a mismatch. The problem could be due to either A or B. Or both.
Both should be worked on - at least until one can be fully ruled out.