In the CBS sitcom “The Big Bang Theory,” fictional scientists Sheldon Cooper and Leonard Hofstadter spent numerous episodes jotting down equations on whiteboards, attempting to determine how fusion reactors could potentially generate axions—speculative particles associated with dark matter. Their efforts were unsuccessful. Now, a genuine group of physicists claims to have resolved the challenge that the sitcom’s creators could not.
Researchers from the University of Cincinnati, Fermilab, MIT, and Technion-Israel Institute of Technology have released a theoretical paper in the Journal of High Energy Physics indicating that upcoming fusion reactors may serve as powerful dark matter detectors. The breakthrough hinges on leveraging something overlooked by the TV physicists: the significant surge of neutrons produced by fusion processes.
Dark matter constitutes approximately 85 percent of the universe’s mass, yet it has never been directly detected. Scientists confirm its existence due to its gravitational influence on galaxy movement and the universe’s development post-Big Bang. A prominent theory proposes that dark matter is made up of extremely light particles known as axions or axion-like particles, which interact minimally with ordinary matter.
What the Sitcom Missed
The fictional equations portrayed in the show, briefly shown on whiteboards, indicated axions produced in the Sun. Jure Zupan, the University of Cincinnati physicist leading the new research, clarifies that this method encounters limitations. While the Sun generates far more particles than any reactor on Earth, theoretically making solar axions easier to detect, fusion reactors facilitate entirely different production methods.
In deuterium-tritium fusion—the reaction planned for establishments like ITER in France—about 80 percent of the energy is carried away by high-energy neutrons traveling at roughly 14 million electron volts. These neutrons collide with the reactor’s inner walls, which are equipped with lithium-rich “breeding blankets” designed to trap neutrons and produce additional tritium fuel.
According to the research, those neutron collisions could generate dark sector particles through two mechanisms. When neutrons are absorbed by nuclei in the reactor walls, the stimulated nuclei can release exotic particles rather than regular radiation as they revert to a stable state. Neutrons can also scatter and decelerate through a process known as bremsstrahlung, or “braking radiation,” potentially producing axions.
“The general concept from our paper was raised in ‘The Big Bang Theory’ years ago, but Sheldon and Leonard couldn’t make it happen,” Zupan points out.
The neutron flux in a fusion reactor is approximately 100 times greater than in comparable fission reactors, creating conditions conducive to the production of these theoretical particles in detectable quantities. Unlike heat and light, which remain confined within the reactor, dark matter particles would traverse meters of steel and concrete as if those materials were nonexistent.
Transforming Energy Facilities Into Physics Labs
To capture these elusive particles, the team suggests positioning a detector near a large fusion facility. Their model employs a design akin to the Sudbury Neutrino Observatory: a massive tank filled with 1,000 tonnes of heavy water. When a dark particle from the reactor passes through, it might collide with a deuterium nucleus, causing it to disintegrate and generating a detectable signal.
The researchers project that year-long searches at future reactors could investigate axion interactions beyond the current experimental thresholds. The high energy of fusion neutrons offers the necessary “boost” to produce exotic particles in ways that previous nuclear technologies simply cannot replicate.
As the first wave of fusion reactors progresses towards operation in the 2030s, these machines could fulfill dual roles. While their main objective remains the generation of clean energy for the electrical grid, integrating dark matter sensors would allow scientists to investigate fundamental physics at virtually no additional expense. The same technology designed to mimic the Sun on Earth may also uncover the true composition of most of the universe.
In essence, the machines engineered to address our energy challenges could also resolve one of cosmology’s most significant questions—a solution that the fictional physicists never fully attained.
[Journal of High Energy Physics: 10.1007/JHEP10(2025)215](https://doi.org/10.1007/JHEP10(2025)215)
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