  The discrepancy between the mass of galaxies and their rotational velocity is
one of the most puzzling scientific phenomena. Despite over a century of
research, this phenomenon is not fully understood. Common explanations include
dark matter and MOND, among other theories. Here we report on another
observation that shows tension between the physics of galaxy rotation and its
rotational velocity. We compare the brightness of galaxies, and find that
galaxies that spin in the same direction as the Milky Way have different
brightness than galaxies that spin in the opposite direction. While such
difference in brightness is expected due to Doppler shift, it is expected to be
subtle. The results show that the difference in brightness is large enough to
be detected by Earth-based telescopes. That observed difference corresponds to
physical properties of galaxies with far greater rotational velocity than the
rotational velocity of the Milky Way. The difference is consistent in both the
Northern galactic pole and the Southern galactic pole, and is not observed in
parts of the sky that are perpendicular to the galactic pole. The differences
are observed by several different instruments such DECam, SDSS, Pan-STARRS, and
HST. The observation is also consistent across annotation methods, including
different computer-based methods, manual annotation, or crowdsourcing
annotations through Galaxy Zoo, all show similar results. Another possible
explanation to the observation is parity violation in the large-scale
structure, such that the magnitude of the parity violation was stronger in the
earlier Universe. It can also be linked to other anomalies such as the Ho
tension. Analysis of Ho using Ia supernovae shows smaller Ho tension when the
spin directions of the host galaxies are consistent, although these results are
based on a small number of supernovae, and may not be statistically
significant.
