Imagine a universe hiding secrets so profound that they could rewrite the very laws of physics—dark matter, the enigmatic substance that makes up most of our cosmos, might just be playing by the rules of gravity after all. But here's where it gets controversial: could this discovery finally bury the idea of a fifth fundamental force, or is there still room for a cosmic wildcard?
Scientists have uncovered that dark matter, the universe's most elusive and mysterious component, adheres to the pull of gravity on enormous scales across the universe. This breakthrough could effectively eliminate the prospect of a fifth fundamental force of nature—but even if it doesn't fully extinguish it, it definitely clamps down on how powerful such a force could be.
Let's break this down a bit for those new to the topic. We all know that 'normal' matter—the stuff we see and touch—is built from atoms, which themselves consist of protons, neutrons, and electrons. These everyday particles interact through the four established fundamental forces: electromagnetism, gravity, the strong nuclear force, and the weak nuclear force. For a quick example, think of electromagnetism as the force that lets magnets stick to your fridge or lights your home, while gravity is what keeps your feet on the ground. But dark matter? It's been a puzzle because it doesn't engage with light or electromagnetic radiation like regular matter does—and if it does, it's incredibly faint. That means it's invisible to our eyes and telescopes, detectable only by its gravitational tug, which acts like a puppet master, bending the paths of light and ordinary matter around it.
To crack this mystery and check if dark matter might be influenced by a previously unknown fifth force, a team from the University of Geneva (UNIGE) investigated whether dark matter plunges into cosmic gravity wells just like regular matter on vast scales. Picture gravity wells as massive dents in the universe's fabric—space and time woven together in a four-dimensional tapestry called spacetime, as described by Einstein's general relativity. Heavier objects create deeper wells, pulling things in with stronger force. For instance, a black hole's gravity well is so deep that not even light can escape, while planets orbit in shallower ones around stars.
'As a way to test this, we examined how fast galaxies move compared to the depth of these gravitational wells,' explained Camille Bonvin, a UNIGE researcher. 'If dark matter isn't affected by a fifth force, galaxies—mostly composed of dark matter—should tumble into these wells just like ordinary matter, following only gravity's lead.
But if there's a fifth force at play, it would alter galaxy movements, making them behave differently in the wells. So, by matching well depths to galaxy speeds, we can probe for any such extra influence.'
Using the latest cosmological data, the team confirmed that dark matter indeed slides into these gravity wells in the same way as regular matter. This finding doesn't hint at a fifth force, though it doesn't completely rule one out.
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'At this point, our results don't exclude an unknown force entirely,' noted Nastassia Grimm, the team leader and a researcher at the Institute of Cosmology and Gravitation at the University of Portsmouth in the UK. 'However, if such a fifth force does exist, it can't be stronger than 7% of gravity's power—or we'd have spotted it in our studies.'
These outcomes don't slam the door shut on a potential fifth force, but they sharpen our understanding of dark matter's nature. If there is such a force, it probably won't stay hidden much longer.
'New data from upcoming experiments like LSST, the Legacy Survey of Space and Time at the Vera C. Rubin Observatory, and DESI, the Dark Energy Spectroscopic Instrument, will detect forces as feeble as 2% of gravity,' said Isaac Tutusaus, a team member from the University of Toulouse. 'This means we'll uncover even more about dark matter's behavior.'
The study appeared in Nature Communications on Monday, November 3. (For the full paper, check out: https://go.redirectingat.com/?id=92X1588396&xcust=spaceus1996348822069747901&xs=1&url=https%3A%2F%2Fwww.nature.com%2Farticles%2Fs41467-025-65100-8&sref=https%3A%2F%2Fwww.space.com%2Fastronomy%2Fdark-universe%2Fdark-matter-obeys-gravity-after-all-could-that-rule-out-a-5th-fundamental-force-in-the-universe)
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Robert Lea is a science journalist based in the U.K., with pieces in outlets like Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek, and ZME Science. He also contributes to Elsevier and the European Journal of Physics on science communication. Rob earned his bachelor of science in physics and astronomy from the U.K.'s Open University. Follow him on Twitter @sciencef1rst.
But let's stir the pot a bit: what if dark matter's obedience to gravity is just the tip of the iceberg, and a fifth force is out there, subtly shaping the cosmos in ways we can't yet measure? Do you think this discovery settles the debate, or does it open up new avenues for radical theories? Some might argue that clinging to the four forces is too conservative—after all, science has evolved by questioning the status quo. What are your thoughts? Agree or disagree? Jump into the comments below and let's discuss!
