Movement doesn't make sense when we can't predict or explain it. Based on our understanding of gravity as confirmed by observations of our solar system we expect to see galaxies do X but instead they do Y, and then we collectively fail to identify a simple explanation like bad math or a bad assumption.
In one sense this is a vindication of our application of the scientific method and the way we make theories: a bad theory wouldn't be able to be checked, whereas a good theory can make precise enough claims that when a limitation is found (such as when our predictions about reality do not match our observations) that the results of the check are clear.
Being able to precisely calculate movement of planets in Solar system and also calculate their mass was a huge triumph of physics. The problem is the same math doesn't work with visible stars orbiting in galaxies nor galaxy clusters. Simplest explanation is there's much invisible mass - dark matter. Or the laws are different at these scales.
They greatly simplify models, otherwise they’re too complicated to calculate.
So they simplify the data points, assume point particles, assume no interactions due to electromagnetism, no tidal locks, and Newtonian gravity instead of relativity.
And then it turns out galaxies sometimes rotate too quickly.
Yeah, no shit. If your data is known to be wrong and your model uses the wrong theory of gravity and makes known false simplifications it would be quite strange if it somehow did predict without some discrepancies
Explain for a layman? I don't know what it means for movement to not make sense.