Why are M dwarfs larger and cooler than models predict?
In plain words
Small red stars called M dwarfs, measured precisely in eclipsing pairs, are a few percent larger and cooler than models say. Strong magnetic fields or dark starspots may be the cause, but this is not established.
Precise statement
Low-mass stars with $M\sim 0.1$ to $0.7\,M_{\mathrm{sun}}$ in eclipsing binaries have radii approximately 3 to 10 percent larger and $T_{\mathrm{eff}}$ about 3 to 5 percent lower than standard models at fixed mass. Determine the mechanism (magnetic suppression of convection, starspot coverage, metallicity or opacity errors) and how the inflation depends on magnetic activity, in particular whether it persists in slowly rotating, inactive single M dwarfs.
What would settle it
Radius measurements of slowly rotating single M dwarfs and wide binaries at the 1 percent level, compared with magnetoconvection models of the same stars.
Status in the literature
Single rapidly rotating fully convective M dwarfs are also inflated (Kesseli et al., AJ, 2018), while some long-period eclipsing binaries agree with nonmagnetic models (Han, Muirhead and Swift, AJ, 2019).