Do observed bar speeds agree with dark-matter friction predictions?
In plain words
A rotating bar of stars in a galaxy center should be braked by the surrounding dark matter. Many observed bars still turn fast, which may conflict with the standard dark-matter model.
Precise statement
Dynamical friction with a cold dark matter halo should slow bars to $R = R_{\mathrm{CR}}/R_{\mathrm{bar}} > 1.4$ ($R_{\mathrm{CR}}$ is the corotation radius), while most measured bars have $R < 1.4$. Decide whether observed bar pattern speeds and their evolution, including the Milky Way bar's slowing inferred from the kinematics and metallicity of stars trapped at its corotation resonance, which include the Hercules moving group (Chiba, Friske and Schoenrich 2021; Chiba and Schoenrich 2021), are consistent with Lambda-CDM halos, and if not whether measurement biases or modified halo physics explain the gap.
What would settle it
Pattern speeds for a large sample of barred galaxies with homogeneous methods, compared with cosmological simulations at matching resolution.