Cosmology’s Dark Matter Dilemma: Embracing a Modified Reality?

Modern cosmology leans heavily on the ΛCDM model, wherein nearly 85% of the universe’s matter is attributed to hypothetical dark matter. This mysterious entity is thought to hold galaxies together, form clusters, and drive the growth of large cosmic structures. Yet, despite decades of searching, no dark matter particles have been detected in labs. This conundrum leads some scientists to consider alternatives: perhaps the issue lies not in the ‘invisible substance’ itself, but in how gravity functions. One such alternative is the STVG theory, or MOG-Modified Gravity-where the force of attraction varies with scale. In this model, gravity is strengthened over large distances while remaining near-Newtonian on smaller scales, causing galaxies and clusters to move as if they contain additional mass, though they don’t actually have it. The study’s authors demonstrated that in this theory, nearly all the primary successes of the standard model can be replicated. Galaxy rotation curves, gravitational lensing, cluster behaviors, and relic radiation structures appear similar to those in the ΛCDM model. Mathematically, it seems as though the enhanced gravity fully ‘replaces’ dark matter’s contribution. Consequently, an observational degeneration arises: standard cosmological data cannot distinguish whether real dark matter or enhanced gravity is at play. Density fluctuation growth, parameters, and matter distribution in the universe align in both theories. To telescopes and satellites, these scenarios look virtually the same.

Separately, researchers have analyzed baryonic acoustic oscillations-the characteristic ‘waves’ in galaxy distribution. In the standard model, their shape is explained by dark matter’s influence. However, in MOG, similar smoothing occurs naturally because gravity operates differently across scales. Thus, the observed matter spectrum again fails to differentiate between the two theories. Discrepancies emerge only at the largest scales-billions of light-years across. In the ΛCDM model, gravitational potentials quickly weaken at these sizes, and large-scale coordinated matter movements should be feeble. In MOG, conversely, the enhanced gravity supports these flows, creating noticeable accelerations at gigaparsec scales. Herein lies the ‘crucial test,’ according to the authors. Recent years have uncovered dipole anisotropies in radio galaxy and quasar surveys, exceeding standard model predictions, suggesting powerful galaxy flows. In ΛCDM, such data remains a problem, whereas modified gravity explains them naturally. A further argument in favor of an alternative theory comes from recent observations by the DESI project. These relate to the so-called S8 tension-the discrepancy between expected and observed matter clustering amplitude. MOG aligns better with these measurements without requiring new particles. The authors emphasize this is not an immediate call to abandon dark matter. However, their results show current data do not yet prove its existence as a physical substance. Resolving this debate, in their view, will hinge on future measurements of large-scale cosmic anisotropy. These may reveal whether dark matter is a real form of matter-or merely a reflection of more complex gravitational laws.