  Observed scaling relations in galaxies between baryons and dark matter global
properties are key to shed light on the process of galaxy formation and on the
nature of dark matter. Here, we study the scaling relation between the neutral
hydrogen (HI) and dark matter mass in isolated rotationally-supported disk
galaxies at low redshift. We first show that state-of-the-art galaxy formation
simulations predict that the HI-to-dark halo mass ratio decreases with stellar
mass for the most massive disk galaxies. We then infer dark matter halo masses
from high-quality rotation curve data for isolated disk galaxies in the local
Universe, and report on the actual universality of the HI-to-dark halo mass
ratio for these observed galaxies. This scaling relation holds for disks
spanning a range of 4 orders of magnitude in stellar mass and 3 orders of
magnitude in surface brightness. Accounting for the diversity of rotation curve
shapes in our observational fits decreases the scatter of the HI-to-dark halo
mass ratio while keeping it constant. This finding extends the previously
reported discrepancy for the stellar-to-halo mass relation of massive disk
galaxies within galaxy formation simulations to the realm of neutral atomic
gas. Our result reveals that isolated galaxies with regularly rotating extended
HI disks are surprisingly self-similar up to high masses, which hints at
mass-independent self-regulation mechanisms that have yet to be fully
understood.
