Previously unexplained galaxy rotation was addressed with detection of giant black holes at the centers of galaxies.
They’ve theorized those black holes existed at the center for long before they detected them, and that doesn’t really fix the problem. In fact, since black holes are massive, they would only make the center spin faster, which makes the discrepancy that much worse.
The problem is that the outer arms of the galaxy are spinning too fast, not that the center is spinning too slowly.
Except we still have never actually seen these alleged black holes and black holes don’t solve the galaxy rotation problem. It’s theoretical construct piled on theoretical construct, a series of fudges to explain why the theory doesn’t fit the observations.
At what point is it permissible to admit that the theory is just plain wrong?
No. The fact that galaxies rotate is thus explained, but the sped at which stars revoke about the central black hole can’t be explained by Newtonian gravity or GR. Basically, the speed depends on the mass of the central body. The reverse is true; knowing the orbital speed allows calculation of the central mass, assuming the orbital distance is known. The mass calculated this way should be independent of which orbiting body is used. For galaxies this is not the case. Stars further out orbit faster than predicted. This is only possible if there is unaccounted for mass or GR is wrong. Hence either dark matter or MOND. Dark matter works better in cases other than galactic rotation, so that’s the accepted explanation.