For decades, scientists have tried to solve one of the biggest puzzles in physics: the nature of dark matter.
Although it makes up most of the matter in the universe, no one knows what it actually is.
Popular candidates such as axions and WIMPs (weakly interacting massive particles) have been hunted for more than 40 years, but none have ever been found.
Now, researchers are turning to a very different possibility: the gravitino.
This exotic particle, predicted by a mathematical framework that tries to unify particle physics with gravity, could provide a radically new explanation for dark matter.
The gravitino comes from a theory called N=8 supergravity, first proposed in the late 1970s.
Nobel Prize–winning physicist Murray Gell-Mann noticed in 1981 that this theory seemed to naturally include all the known particles of the Standard Model—quarks and leptons—though with a slight mismatch in electric charge.
Years later, physicists Krzysztof Meissner of the University of Warsaw and Hermann Nicolai of the Max Planck Institute for Gravitational Physics revisited the idea.
They found a way to modify it so that the charges of quarks and leptons came out correctly, making the theory more consistent with reality.
This modification led to a surprising prediction: that gravitinos, particles with spin 3/2, could exist in different charged states.
They would be unimaginably heavy—close to the Planck mass, about a billion-billion times the mass of a proton. Six of them would carry charges of ±1/3, and two would carry ±2/3. Unlike other particles, they cannot decay because there’s nothing lighter for them to decay into. That stability makes them potential candidates for dark matter.
At first glance, it seems strange to think of dark matter as charged. Normally, dark matter is assumed to be electrically neutral so that it doesn’t interact with light. But gravitinos could still qualify as “dark” because they are so rare.
In our solar system, there may be only about one gravitino per 10,000 cubic kilometers of space. Their scarcity means they don’t shine or reveal themselves in the ways charged particles usually would.
The challenge now is to detect them. Meissner and Nicolai have suggested that neutrino detectors—giant underground facilities designed to capture faint flashes of light from ghostly neutrinos—might also be able to spot gravitinos.
In particular, the Jiangmen Underground Neutrino Observatory (JUNO) in China, set to begin taking data in 2025, could be well-suited for the task.
JUNO is a massive experiment: a spherical detector 40 meters across, filled with 20,000 tons of synthetic liquid, and lined with more than 17,000 light sensors. Its main goal is to study the properties of neutrinos, but its enormous size also makes it sensitive to extremely rare events.
In a recent study published in Physical Review Research, Meissner, Nicolai, and their collaborators used advanced simulations combining particle physics and quantum chemistry to predict what a gravitino passing through JUNO would look like.
Their analysis accounted for possible sources of noise, from natural radioactivity in the detector liquid to the sensitivity of the sensors. The results suggest that a gravitino’s signal would be unique—so distinctive that it could not be mistaken for any known particle.
The implications are enormous. If JUNO or future detectors such as the Deep Underground Neutrino Experiment (DUNE) in the United States were to catch even a single gravitino, it would provide the first experimental evidence for physics at the Planck scale.
That would bring us closer than ever to a unified theory of nature, one that combines gravity and particle physics into a single framework.
For now, the search continues. Dark matter remains elusive, but the gravitino may offer a fresh and daring path forward—one that could change our understanding of the universe itself.
