Zachary Weiner has spent years studying something invisible — dark matter, the mysterious substance that makes up about 27% of the universe but has never been directly detected. As a researcher at the Perimeter Institute for Theoretical Physics in Canada, Weiner and his team have just uncovered something that defies common sense: dark matter might have its own secret force, but that force works in a surprising way.
Scientists have long believed dark matter only interacts with ordinary matter through gravity. Gravity is what holds planets, stars, and galaxies together. But what if dark matter particles also pull on each other through a hidden "dark force" that we cannot detect?
Weiner's team modeled what would happen if dark matter experienced an extra attractive force alongside gravity. Their study, published in the Journal of Cosmology and Astroparticle Physics, found that this force does make dark matter clump together more easily. You might expect that would speed up the formation of galaxies and other cosmic structures. Instead, the opposite happens — the growth of large structures actually slows down.
The reason is counterintuitive. When dark matter particles attract each other more strongly, they also gradually become effectively lighter as the universe expands. Think of it like a balloon: when you stretch it, the rubber gets thinner even though there's still the same amount of material. This loss of "weight" weakens dark matter's gravitational pull, canceling out the benefits of stronger clustering.
"The first thing you would expect is that giving dark matter an additional attractive force should make structures grow faster," Weiner said. "But another effect comes into play at the same time."
This discovery matters for real observations. The Dark Energy Spectroscopic Instrument, or DESI, has been mapping millions of galaxies to understand how the universe expands. Some explanations for DESI's measurements involve interactions between dark matter particles. Weiner's findings suggest those models need to account for dark matter becoming lighter over time.
The discrepancies driving this research are small but intriguing. Observations of the distant universe suggest cosmic expansion may have been slightly slower in the past than standard models predict. Meanwhile, studies of the cosmic microwave background — ancient light from the early universe — hint that matter might be more densely packed across vast scales than expected. Scientists want to know if a hidden force could explain these puzzles.
More precise measurements from upcoming telescopes and surveys could finally reveal which hidden interactions dark matter truly possesses. "The Universe is often more subtle than our intuition," Weiner said. "That's exactly why we have to keep testing these ideas."
For Weiner, the surprise itself is the point. The universe rarely behaves the way first instincts suggest — and that mystery is precisely what makes exploration worthwhile.