Why giving dark matter an additional attractive force does not necessarily produce “clumpier matter.”
Dark matter shapes galaxies and the large structure of the Universe through gravity, yet no one knows whether gravity is its only influence. Dark matter particles might also pull on one another through a hidden force that ordinary matter cannot feel.
Researchers explored that possibility in the Journal of Cosmology and Astroparticle Physics (JCAP) and reached a counterintuitive conclusion. An additional attractive force can bring dark matter particles closer together without accelerating the overall growth of cosmic structure. Under most conditions examined, it actually suppresses that growth.
The idea has gained attention because increasingly precise measurements do not all produce a perfectly consistent picture of the Universe. Observations of cosmic expansion and measurements of how galaxies and larger structures developed over time show small but persistent differences.
Some studies of the distant Universe indicate that expansion may have proceeded slightly more slowly in the past than the standard cosmological model predicts. Meanwhile, observations of the cosmic microwave background, the leftover radiation from the early Universe, have suggested that matter could be more strongly clustered across the largest scales than expected.
These disagreements remain modest, but they have encouraged physicists to test whether the standard model is missing some property of dark matter or another cosmic ingredient.
A hidden force could fill the gap
One possibility is a “dark force” that operates only between dark matter particles. Because such an interaction could alter both cosmic expansion and the formation of galaxies and other structures, it might help account for some of the observational tensions.
“What we really know about dark matter has so far been learned only through its gravitational effects,” says Zachary Weiner, a researcher at the Perimeter Institute for Theoretical Physics, corresponding author for the study. “That leaves open the possibility that dark matter might have additional interactions that are hidden from ordinary matter.”
Weiner and his colleagues examined theoretical models in which dark matter experiences a long-range attractive force in addition to gravity.
They combined theoretical calculations with cosmological observations to trace two connected effects: how the added interaction changes the expansion history of the Universe and how it influences the growth of large-scale structure.
More attraction does not mean more structure
The simplest expectation is that another attractive force would make dark matter gather more quickly. Faster clustering might then produce denser cosmic structures and potentially explain observations suggesting that matter is more concentrated than standard predictions allow.
“The first thing you would expect is that giving dark matter an additional attractive force should make structures grow faster,” says Weiner. “But another effect comes into play at the same time.”
The hidden force does increase the attraction among dark matter particles. That is only one part of the calculation, however, and the second effect changes the final outcome.
Dark matter becomes lighter over time
Within the models the researchers examined, the same interaction that strengthens clustering also changes dark matter as the Universe expands. The particles effectively lose mass over time.
That declining mass weakens their gravitational influence. Although the particles gather more efficiently because of the additional force, their reduced gravitational impact outweighs the extra clustering.
Consequently, the interaction does not strengthen dark matter’s imprint on the cosmic microwave background. In most of the scenarios studied, it instead slows the overall development of cosmic structure.
The finding constrains broader theories
The result could affect theories extending beyond the simplest dark force models. Several explanations proposed for recent measurements from the Dark Energy Spectroscopic Instrument (DESI) include related interactions among dark matter particles.
According to the researchers, the same competition between stronger attraction and decreasing effective mass is likely to operate in many of those more elaborate models. Any attempt to use dark matter interactions to explain cosmological observations must therefore account for both effects rather than assuming that added attraction automatically creates more structure.
New surveys and observatories are producing measurements with steadily improving precision. Comparing those observations with predictions from models like these may narrow the range of hidden interactions dark matter can—and cannot—have.
“The Universe is often more subtle than our intuition,” says Weiner. “That’s exactly why we have to keep testing these ideas.”
Reference: “Dark forces suppress structure growth” by Marco Costa, Cyril Creque-Sarbinowski, Olivier Simon and Zachary J. Weiner, 22 June 2026, Journal of Cosmology and Astroparticle Physics.
DOI: 10.1088/1475-7516/2026/06/055
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