Dark matter is usually described as an invisible substance that responds only to gravity. But scientists are now exploring a more complex possibility: dark matter particles may also interact through a hidden force of their own.
A new study in the Journal of Cosmology and Astroparticle Physics (JCAP) examined what would happen if dark matter experienced an additional attractive force. The results were surprisingly counterintuitive. Although the force causes dark matter to gather more efficiently, it usually does not accelerate the growth of cosmic structure. Instead, it tends to slow that growth.
Why Scientists Are Considering a Dark Force
Interest in a possible “dark force” has increased as highly precise observations of the Universe have produced results that do not always align perfectly.
Measurements of cosmic expansion and the development of galaxies and other large structures sometimes appear to tell slightly different stories. Some observations of the distant Universe suggest that expansion may have proceeded somewhat more slowly in the past than the standard cosmological model predicts.
At the same time, studies of the cosmic microwave background have long suggested that matter could be more tightly clustered across the largest scales of the Universe than expected.
These differences are relatively small, but they have encouraged scientists to ask whether the standard model of cosmology may be missing an important ingredient.
One possibility is that dark matter particles feel an extra force that ordinary matter cannot detect. Because the interaction would operate only within the dark matter sector, researchers refer to it as a “dark force.” Such an interaction could potentially affect both the Universe’s expansion and the formation of galaxies and larger cosmic structures.
“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.”
Testing a Force Beyond Gravity
The research team investigated a group of theoretical models in which dark matter particles interact through a long-range force in addition to gravity.
Using theoretical calculations together with cosmological data, the scientists studied how this hidden interaction would influence the history of cosmic expansion and the growth of large-scale structure.
At first, the expected result seems straightforward. If dark matter particles attract each other through another force, they should assemble into clumps more quickly. That stronger clustering might also appear to explain observations suggesting that the Universe contains denser structures than predicted.
“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.”
A Surprising Effect on Cosmic Growth
In the models examined by the researchers, the extra force does make dark matter cluster more effectively. However, it also changes how dark matter behaves as the Universe expands.
The same process causes dark matter particles to effectively lose mass over time. This reduction weakens their gravitational influence, offsetting the stronger attraction produced by the hidden force.
As a result, the enhanced clustering does not create a stronger gravitational imprint on the cosmic microwave background. In most cases, the combined effect actually suppresses the growth of cosmic structure.
Implications for DESI and Dark Energy Models
The findings could matter for theories that extend beyond dark matter alone. Some explanations proposed for recent measurements from the Dark Energy Spectroscopic Instrument (DESI) involve similar interactions among dark matter particles.
According to the researchers, the newly identified mechanism is likely to influence many of those more complicated models as well. Any theory involving a hidden attractive force may need to account for the possibility that dark matter becomes effectively lighter as the Universe evolves.
More precise measurements from upcoming observatories and cosmic surveys could help scientists determine which hidden interactions dark matter might possess and which possibilities observations can rule out.
“The Universe is often more subtle than our intuition,” says Weiner. “That’s exactly why we have to keep testing these ideas.”