The enigmatic nature of dark matter has long perplexed physicists and astronomers alike. Despite constituting approximately five times more mass than ordinary, baryonic matter in the cosmos, this elusive substance neither emits nor reflects light, rendering it effectively invisible to direct observation. The fundamental question remains: Does dark matter obey the same physical laws as the particles described by the Standard Model, or is it influenced by unknown forces that transcend current theoretical frameworks? A recent investigation undertaken by an international collaboration, prominently featuring researchers from the University of Geneva (UNIGE), has taken a pivotal step in unraveling this cosmic mystery. Their findings, published in the prestigious journal Nature Communications, indicate that dark matter behaves in a manner consistent with conventional gravitational laws, yet they leave the door ajar for subtle deviations that could hint at new physics.
Central to understanding these results is the role of gravity as it manifests on cosmological scales. Ordinary matter, composed of atoms and molecules, gravitates toward regions of dense mass, forming structures such as stars, galaxies, and clusters. This clustering arises because space-time itself is curved by mass-energy, creating gravitational wells into which matter naturally falls. Einstein’s general theory of relativity provides the mathematical framework to describe how gravity shapes the universe at large. Complementarily, classical fluid dynamics, encapsulated in Euler’s equations, governs how ordinary matter’s velocity fields respond to these potential wells. Whether dark matter conforms to the same hydrodynamic principles has been a subject of intense debate, with implications that stretch to the core of particle physics and cosmology.
In this groundbreaking study, the UNIGE-led team sought to directly evaluate whether dark matter exhibits motion analogous to ordinary matter under the influence of these gravitational potentials. The methodology capitalized on examining the velocities of distant galaxies, which serve as tracers predominantly composed of dark matter halos enveloping visible structures. If dark matter interacts solely through gravity, then galaxies’ movements should align with predictions from Euler’s equations within the warped space-time fabric. Conversely, should a hypothetical fifth force act exclusively on dark matter, this would induce measurable deviations in the galactic velocity profiles relative to the gravitational well depths.
Their analysis involved a meticulous comparison between the observed velocities of galaxies and the inferred gravitational potential wells mapped by large-scale surveys. Using state-of-the-art cosmological data, including redshift measurements and gravitational lensing effects, the researchers reconstructed the depth of these wells across vast cosmic distances. The results revealed a remarkable concordance: dark matter-dominated galaxies fall into gravitational wells with dynamics consistent with Euler’s hydrodynamic equations and the predictions of general relativity. This outcome suggests that, at least within current observational limits, dark matter experiences gravity in much the same way as ordinary matter.
Nonetheless, the study does not entirely dismiss the possibility of dark matter being influenced by additional forces. According to Nastassia Grimm, the first author and former postdoctoral scholar at UNIGE now affiliated with the University of Portsmouth, any such fifth force must be extremely feeble—less than 7% the strength of gravity—otherwise its effects would have surfaced in the velocity-depth comparisons. This upper boundary places tight constraints on speculative models proposing new interactions within the dark sector, effectively narrowing the landscape of viable dark matter theories.
The implications of these findings are profound for both theoretical physics and observational cosmology. Firstly, affirming that dark matter conforms to Euler’s equations across cosmological scales bolsters the foundational assumptions underpinning large-scale structure formation models. These models simulate how primordial fluctuations evolved into the cosmic web of galaxies observed today. Secondly, the constraints on fifth forces guide particle physicists in refining dark matter candidates, from weakly interacting massive particles (WIMPs) to axions and beyond, ensuring such models remain consistent with astrophysical observations.
Looking forward, the quest to further elucidate dark matter’s nature hinges on upcoming experimental and observational campaigns. Notably, next-generation surveys like the Legacy Survey of Space and Time (LSST) conducted by the Vera C. Rubin Observatory, alongside the Dark Energy Spectroscopic Instrument (DESI), promise unprecedented sensitivity to subtle forces on dark matter. These instruments will scrutinize galaxy clustering and velocity fields with exquisite precision, potentially detecting fifth forces as weak as 2% the strength of gravity. Such capabilities could herald a paradigm shift, unveiling new interactions that have so far eluded detection.
The study also highlights the indispensable synergy between theoretical modeling and empirical data in contemporary cosmology. By directly confronting hypotheses about dark matter dynamics with rigorous observational tests, the scientific community progressively sharpens its understanding of the dark sector’s fundamental characteristics. Camille Bonvin, associate professor at UNIGE and co-author of the paper, emphasized this approach’s elegance: by measuring galaxy velocities relative to gravitational wells, researchers are effectively probing the very fabric of cosmological physics, turning an invisible component into a measurable entity through its dynamical signature.
Moreover, these results underscore the robustness of general relativity as the prevailing theory of gravity, even amid the Universe’s mysterious constituents. While alternative gravitational theories and dark sector interactions remain intriguing, the current evidence affirms that, at the scales investigated, gravity reigns supreme in orchestrating cosmic structure formation. This affirmation does not diminish the allure of dark matter’s unknown qualities but rather frames the scientific challenge with greater clarity.
In conclusion, the latest research led by the University of Geneva marks a significant leap in constraining dark matter’s physical laws. While dark matter appears to fall into gravitational wells just like ordinary matter, the search for extraordinary phenomena governing this unseen majority continues. The stringent limits established on potential non-gravitational interactions narrow the theoretical playground and motivate the exploitation of forthcoming data to probe even more subtle effects. As the next decade of cosmological observations unfolds, the scientific community edges closer to unveiling the true nature of dark matter—an endeavor that stands to revolutionize our comprehension of the Universe at its most fundamental level.
Subject of Research: Not applicable
Article Title: Does dark matter fall in the same way as standard model particles? A direct constraint of Euler’s equation with cosmological data
News Publication Date: 3-Nov-2025
Web References: 10.1038/s41467-025-65100-8
Keywords
Dark Matter, Cosmology, Euler’s Equations, Gravitational Wells, Fifth Force, Galaxy Velocities, General Relativity, Large-Scale Structure, LSST, DESI, Cosmological Data, Universe
