About 85 percent of the matter in the universe is thought to be dark matter, yet there is still no confirmed direct detection of any dark matter particle. Ground-based detectors, space-based telescopes, and the Large Hadron Collider near Geneva have so far not produced a definitive signal. That long-running lack of confirmation has kept alive an uncomfortable question: what if the missing mass is not made of hidden particles at all, but instead reflects a gap in our understanding of gravity?
Why Dark Matter Particles Remain Elusive
The standard cosmological model treats dark matter as a sea of slow-moving, weakly interacting particles that clump around galaxies and guide the growth of cosmic structure. The framework works remarkably well at large scales, predicting the pattern of temperature fluctuations in the cosmic microwave background and the distribution of galaxy clusters across billions of light-years. But the particle itself remains a ghost. Scientists have searched for it with underground xenon detectors, gamma-ray satellites, and high-energy collisions, according to reporting in the popular press, and every hunt has come up empty.
That absence carries real scientific weight. If dark matter particles exist, they should interact with ordinary matter at some measurable rate, however faint. The failure to find that signal after more than three decades of increasingly sensitive experiments has pushed some physicists to revisit an older idea: perhaps the laws of gravity themselves need revision. The scientific community still broadly prefers pursuing the dark matter hypothesis, which has been judged superior in terms of pursuitworthiness when weighed against modified gravity, because it meshes smoothly with the wider edifice of particle physics and cosmology. But “pursuitworthy” is not the same as confirmed, and the alternative camp has been producing sharper theoretical tools.
MOND and the Radial Acceleration Puzzle
Modified Newtonian Dynamics, or MOND, was first proposed in the 1980s. The core claim is simple: gravitational forces become stronger than Newton predicted when accelerations drop below a specific threshold, roughly one ten-billionth of a meter per second squared. That single adjustment, with no added particles, can reproduce the flat rotation curves that galaxies display at their edges, the very observation that originally motivated dark matter. A peer-reviewed study reported a tight empirical relation across 153 galaxies and thousands of individual radial data points, showing that the observed acceleration tracks the acceleration predicted by visible baryonic mass alone. The correlation is so clean that some researchers describe it as evidence that galaxies “know about their baryons,” a pattern that particle dark matter models must work hard to explain.
The trouble is that MOND, in its original nonrelativistic form, cannot account for gravitational lensing or the behavior of the universe at cosmological scales. Jacob Bekenstein addressed the lensing problem by constructing a relativistic tensor–vector–scalar theory, often abbreviated TeVeS, which was formulated in a detailed theoretical treatment designed to reproduce MOND-like behavior while handling the bending of light around massive objects. More recently, Constantinos Skordis and Tom Zlosnik proposed a newer relativistic theory that goes further: it matches galactic MOND behavior and also agrees with key linear-cosmology observables, including the cosmic microwave background and the matter power spectrum. That result directly challenges the long-standing critique that modified gravity falls apart on the largest scales, even if it does not yet command the same breadth of empirical support as the standard dark matter picture.
The Bullet Cluster Problem
If MOND-style theories had a single most damaging piece of counter-evidence, it would be the Bullet Cluster. This system consists of two galaxy clusters that collided and passed through each other. Observations showed a clear spatial separation between the hot X-ray-emitting gas, which makes up most of the ordinary matter, and the regions where gravitational lensing indicates the bulk of the mass resides. That offset is hard to explain without a collisionless mass component that sailed through the collision while the gas was stripped away, exactly the behavior expected of dark matter particles. For many physicists, the Bullet Cluster remains the strongest direct evidence that some form of invisible matter exists independent of ordinary baryons, and it is frequently cited as a key reason to remain cautious about purely modified-gravity explanations.
Modified gravity proponents have not conceded the point, but they have not fully resolved it either. Some hybrid proposals try to split the difference. One framework keeps a dark sector of particles that form a superfluid inside galaxies, producing MOND-like phenomenology through emergent phonon-mediated forces while behaving like conventional dark matter at cluster scales. The approach is creative, but it also concedes the central point that gravity alone may not be enough. It essentially asks whether the “missing piece” is a new kind of particle whose collective behavior mimics modified gravity under specific conditions, rather than a straightforward change to gravitational law. In parallel, detailed studies of other colliding clusters continue to test whether the Bullet Cluster is representative or an outlier, sharpening the constraints on both camps.
Emergent Gravity and the Measurement Trap
A more radical proposal comes from theoretical physicist Erik Verlinde, who argued that gravity may not be a fundamental force at all but rather an emergent effect arising from quantum entanglement and the microscopic structure of spacetime. In this picture, the extra gravitational pull attributed to dark matter is a natural consequence of how information is distributed across space, producing an additional dark gravitational component tied to the acceleration scale a₀, which is approximately the product of the speed of light and the Hubble constant. Verlinde’s emergent gravity framework makes specific scaling predictions that differ from both standard dark matter and traditional MOND, especially in how gravity should behave in galaxy clusters and around isolated galaxies with different mass profiles.
Testing such ideas is challenging because the signals are subtle and astrophysical data are messy. A recent analysis in the journal Nature Astronomy examined weak lensing measurements around galaxies to see whether the inferred gravitational pull follows the patterns expected from particle dark matter or from modified gravity. In their observational study, the authors reported a lensing signal consistent with ordinary matter plus an additional component that behaves like cold dark matter, which they argue constrains simple MOND-like deviations and some emergent scenarios. Yet even these results leave room for more elaborate modified-gravity models that can mimic dark matter over certain ranges while diverging in others, underscoring how difficult it is to design tests that cleanly distinguish between competing explanations.
A Community Feeling Its Way Forward
Behind the technical debate lies a broader question about how physics should respond when a powerful, well-tested framework faces a persistent empirical gap. The dark matter paradigm has scored many successes, from explaining the growth of large-scale structure to fitting the detailed pattern of temperature anisotropies in the cosmic microwave background. Modified gravity, by contrast, shines in explaining galactic rotation curves and the tight relation between baryons and acceleration, but struggles with clusters and cosmology unless carefully extended. The situation has led to a kind of methodological pluralism: many researchers continue to refine particle models and search for signals in underground detectors, while a smaller but active minority develops alternative theories and hunts for telltale deviations in astrophysical data.
That pluralism is reflected in how new work circulates. Preprint servers such as arXiv, which is supported by a broad consortium of institutional members, allow researchers on both sides of the debate to share ideas quickly, receive feedback, and iterate before formal peer review. In practice, the dark matter versus modified gravity discussion now plays out as a sequence of increasingly precise observational challenges: new measurements of lensing, galaxy dynamics, and cosmic structure formation are checked against both particle-based simulations and alternative gravity models. Over time, the hope is that this iterative process will tighten the net enough that one framework, or perhaps a hybrid descendant of both, emerges as the clear winner.
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*This article was researched with the help of AI, with human editors creating the final content.
