Occam's Razor makes us legitimately suspicious of Dark Matter, in amounts radically exceeding known matter, as an explanation for galactic rotation anomalies.

The effect is that we demand a great deal of supporting evidence, which has been accumulating, incrementally, although the particle needed to clinch matters has not yet been identified. Still, without DM, we would need some other way to account for the anomalous rotation and the other oddities since identified and attributed to DM.