Dark matter is usually described as an invisible substance that responds only to gravity. But researchers at the Perimeter Institute for Theoretical Physics have explored a more complex possibility: dark matter particles may also interact through a hidden force of their own — a "dark force" that ordinary matter cannot detect.
In a new study published in the Journal of Cosmology and Astroparticle Physics (JCAP), physicist Zachary Weiner and his colleagues modeled what would happen if dark matter experienced an additional attractive force. The result was surprisingly counterintuitive. Although the extra force makes dark matter clump together more efficiently, it usually does not accelerate the growth of cosmic structure — it tends to slow it down.
"At first glance, the outcome seems obvious. If dark matter particles can attract one another through an additional force, they should clump together more efficiently," says Weiner. "But another effect comes into play at the same time."
The same mechanism that strengthens clustering also causes dark matter particles to effectively lose mass as the Universe expands. That mass loss weakens their gravitational pull, offsetting the stronger attraction from the hidden force. In most cases, the combined effect actually suppresses the growth of galaxies and other large structures rather than boosting it.
The findings matter beyond dark matter itself. Some models proposed to explain recent observations from the Dark Energy Spectroscopic Instrument (DESI) rely on similar interactions among dark matter particles. According to the researchers, the newly identified mechanism is likely to affect many of those more complicated scenarios as well.
Interest in a possible "dark force" has grown because some of the most precise observations of the Universe seem to tell slightly different stories: measurements of cosmic expansion and of structure growth do not always fit together as neatly as the standard cosmological model predicts. While the discrepancies are small, they hint that an ingredient may be missing from the model.
"The Universe is often more subtle than our intuition," says Weiner. "That's exactly why we have to keep testing these ideas."
