Hidden Force in Dark Matter Slows Cosmic Structure Growth

A team of physicists led by a researcher at Canada’s Perimeter Institute for Theoretical Physics calculated what would happen if dark matter particles, in addition to gravity, also felt an extra long range attractive force. The result runs counter to expectations: in most of the scenarios considered, such a force slows the growth of large cosmic structures instead of speeding it up. The paper was published on June 22, 2026, in the Journal of Cosmology and Astroparticle Physics (JCAP), with a press release from publisher Sissa Medialab following in early August.

The lead author is Zachary J. Weiner of the Perimeter Institute, with coauthors Marco Costa, Cyril Creque-Sarbinowski and Olivier Simon. Everything currently known about dark matter has been inferred indirectly, from its gravitational effect on visible matter and on light. The first hints emerged in the 1930s, from measurements of galaxy speeds within clusters, and were confirmed in the 1970s by the rotation curves of spiral galaxies. No detector has ever captured a dark matter particle directly.

“What we actually know about dark matter so far has been learned only through its gravitational effects,” Weiner said in the publisher’s statement. That leaves open the possibility that dark matter has interactions hidden from ordinary matter, in other words a force that acts only within the dark sector.

Why look for an extra force at all

Interest in this so called dark force has grown because increasingly precise measurements do not add up to a fully consistent picture of the universe. Observations of the distant universe suggest that expansion in the past proceeded somewhat more slowly than the standard cosmological model predicts. At the same time, measurements of the cosmic microwave background have for years suggested that matter on the largest scales could be more tightly clumped than expected.

These discrepancies are small but persistent, and they have pushed physicists to check whether the standard model is missing some property of dark matter. An extra force acting only among its particles is one candidate, since it could alter both the history of expansion and the formation of galaxies and clusters.

What exactly the team calculated

The team examined a class of theoretical models in which dark matter particles feel a long range attractive force in addition to gravity, then compared the theoretical calculations with cosmological data on the expansion of the universe and on the distribution of large scale structure. Two related effects were tracked, one on the history of expansion, the other on the growth of structure.

The first effect is the expected one. An extra attractive force does help dark matter particles clump together more efficiently.

“The first thing you’d expect is that giving dark matter an extra attractive force should make structures grow faster,” Weiner said. “But at the same time, another effect comes into play.”

That second effect drives the reversal. The same interaction makes dark matter effectively lighter as the universe expands, and the loss of effective mass weakens its gravitational pull more than the extra clumping helps. The net result is that dark matter does not leave a stronger imprint in the background radiation, but the overall growth of cosmic structures slows down instead.

What this means for the models built around DESI

The finding also touches theories that go beyond the simplest dark force models. Several explanations proposed for recent measurements from the Dark Energy Spectroscopic Instrument, known as DESI, involve similar interactions among dark matter particles. According to the authors, the same tug of war between stronger attraction and declining effective mass is likely at work in those more complex models too, so it cannot be assumed that extra attraction by itself produces more structure.

One earlier observation already narrows the room for such models. The cluster 1E 0657-56, better known as the Bullet Cluster, was imaged in 2006 by combining X-ray and optical data, and it showed that in a collision between two clusters the hot gas lagged behind while the mass traced by gravitational lensing passed through almost undisturbed. That image has since been used as evidence that dark matter interacts weakly with itself in collisions. The new calculation deals with a different kind of interaction, a long range force, and now constrains that kind from another angle, through the growth of structure.

Limits of the result

The paper does not prove that a dark force exists. It is a theoretical calculation within a given class of models, compared against existing cosmological data, and its reach is limited to models of that type. No new particle has been measured, no detector has recorded a signal, and the tensions between expansion measurements and matter clustering measurements remain unresolved.

The result shows what happens to structure growth if such a force is assumed, not what nature actually does. More precise measurements from new sky surveys and observatories have yet to show which hidden interactions dark matter is allowed to have, and which can be ruled out.