Why does galaxy dynamics depend only on the visible matter distribution?
In plain words
In rotating galaxies the actual pull of gravity at each radius can be predicted from the stars and gas alone through one universal curve. That is natural in MOND but surprising if invisible dark matter dominates the mass.
Precise statement
The radial acceleration relation $g_{\mathrm{obs}} = g_{\mathrm{bar}} / (1 - \exp(-\sqrt{g_{\mathrm{bar}} / a0}))$ with $a0 \sim 1.2 \times 10^{-8}\ \mathrm{cm}/\mathrm{s}^{2}$ fits about 150 SPARC rotation curves with small scatter, and the baryonic Tully-Fisher relation $M_{b}$ proportional to $V_{\mathrm{flat}}^{4}$ holds over 5 decades in mass. Explain whether LCDM galaxy formation produces one relation with a single a0 and the observed small scatter, or whether the relation signals modified dynamics or a dark matter coupling to baryons.
What would settle it
Cosmological hydrodynamic simulations reproducing the relation, its scatter and its residual correlations, or a theory that predicts it while also fitting clusters and the CMB.
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