  Galaxies have been observed to exhibit a level of simplicity unexpected in
the complex galaxy formation scenario posited by standard cosmology. This is
particularly apparent in their dynamics, where scaling relations display much
regularity and little intrinsic scatter. However, the parameters responsible
for this simplicity have not been identified. Using the Spitzer Photometry &
Accurate Rotation Curves galaxy catalogue, we argue that the radial
acceleration relation (RAR) between galaxies' baryonic and total dynamical
accelerations is the fundamental $1$-dimensional correlation governing the
radial (in-disk) dynamics of late-type galaxies. In particular, we show that
the RAR cannot be tightened by the inclusion of any other available galaxy
property, that it is the strongest projection of galaxies' radial dynamical
parameter space, and that all other statistical radial dynamical correlations
stem from the RAR plus the non-dynamical correlations present in our sample. We
further provide evidence that the RAR's fundamentality is unique in that the
second most significant dynamical relation does not possess any of these
features. Our analysis reveals the root cause of the correlations present in
galaxies' radial dynamics: they are nothing but facets of the RAR. These
results have important ramifications for galaxy formation theory because they
imply that to explain statistically late-type galaxy dynamics within the disk
it is necessary and sufficient to explain the RAR and lack of any significant,
partially independent correlation. While simple in some modified dynamics
models, this poses a challenge to standard cosmology.
