Is this the end of dark matter? If the paper is accepted for publication, does it mean we don't need the dark matter theory anymore? Or, should I say, it is not dark anymore?
"The Radial Acceleration Relation in Rotationally Supported Galaxies"  Stacy McGaugh, Federico Lelli, Jim Schombert
 A: That perhaps depends what you mean by dark matter. If by that you mean non-baryonic matter, then no, not by a long way. The rotation curves of galaxies are only one of many pieces of evidence for non-baryonic dark matter, including the dynamics of galaxy clusters, gravitational lensing, the cosmic microwave background, baryon acoustic oscillations, the primordial abundances of He and D and so on.
The rotation curves of galaxies only indicate (assuming our understanding of gravity were correct) large quantities of unaccounted for, unseen mass; and actually, that is completely unchanged by this paper.
A: I'll answer the question I believe to be implied: do the results in this paper mean that the rotation curves are explained solely by the visible matter?
And the answer is that no, visible matter alone cannot account for the rotation curves. Figure 3 in the paper plots the observed centripetal acceleration against the centripetal acceleration and this does not follow the line $x=y$. Instead the relation between observed and calculated centripetal acceleration is well fitted by (equation 4 in the paper):
$$ g_\text{obs} = \frac{g_\text{bar}}{1 - e^{-\sqrt{g_\text{bar}/g_\dagger}}} $$
where $g_\text{obs}$ is the centripetal acceleration calculated from the rotation curves and $g_\text{bar}$ is the centripetal acceleration calculated from the observed matter (baryonic) distribution. The parameter $g_\dagger$ is experimentally fitted and the authors do not suggest a physical interpretation for it.
So to explain the rotation curves still requires some extra effect whether it be dark matter or some as yet unidentified effect.
What is surprising is that the result means that if dark matter is present it must have a distribution closely correlated with the distribution of visible matter. This is surprising because there's no obvious reason why this should be so, and it's quite a strong constraint on the distribution of dark matter. Arguably it makes dark matter a less natural explanation of the rotation curves, but it certainly does not rule it out.
I note with some amusement that the result do seem to be naturally explained by the MOND theory, though the authors are at pains to say they are not suggesting that MOND is the explanation.
