Dark matter’s hidden force slows cosmic structure growth, study finds

A new study published in the Journal of Cosmology and Astroparticle Physics (JCAP) on August 2, 2026, reveals that an additional attractive force among dark matter particles, contrary to expectations, tends to slow rather than accelerate the growth of cosmic structure. The research, led by Zachary Weiner of the Perimeter Institute for Theoretical Physics, shows that while such a force makes dark matter cluster more efficiently, it also causes particles to effectively lose mass over time, weakening their gravitational pull and suppressing structure formation.

Scientists have been exploring the idea of a hidden "dark force" because precise cosmological observations sometimes diverge from the standard model. Measurements of cosmic expansion and the cosmic microwave background hint at discrepancies: some data suggest slower past expansion, while others indicate matter is more tightly clustered than predicted. These small but persistent anomalies motivate the search for additional interactions within the dark matter sector that ordinary matter cannot detect.

According to Weiner, the counterintuitive result stems from two competing effects. The extra attractive force indeed enhances clustering, but it simultaneously alters dark matter's behavior as the universe expands, effectively reducing particle masses. This mass loss offsets the stronger attraction, leading to a net suppression of structure growth in most models. The study examined a group of theoretical models where dark matter particles interact via a long-range force alongside gravity, using theoretical calculations and cosmological data.

The findings have implications for theories attempting to explain recent measurements from the Dark Energy Spectroscopic Instrument (DESI), which some models address by invoking similar dark matter interactions. The researchers note that any theory with a hidden attractive force must account for the effective mass loss mechanism. Upcoming observatories and cosmic surveys may provide more precise data to test these possibilities.

Weiner emphasizes the need for continued testing, stating, "The Universe is often more subtle than our intuition." The study was published by Sissa Medialab.