On September 1, 2026, in Tendou, Yamagata Prefecture, Japan, at the TeV Particle Astrophysics Conference, a spokesperson for the LUX-ZEPLIN (LZ) experiment team took the stage, and the physicists in attendance had no idea what they were about to hear, only that the world's largest dark matter detector seemed to have seen something unusual.
After the results were announced, the entire venue, as well as the entire physics community, was abuzz with excitement. The physics community had not been so excited in a long time. The reason was a detector buried nearly a mile underground, which had recorded an unexplainable flash. Had the dark matter that scientists had been searching for for so long finally revealed itself?
An Unexplainable Flash
The story begins three years ago. At 3:22:39 pm on June 16, 2023, a faint flash suddenly appeared one mile underground.
The flash occurs in a gigantic cylindrical tank filled with 10 tons of ultra-pure liquid xenon. This device is the LZ detector. The LZ experiment is currently the world's largest direct dark matter detection project, involving 250 scientists and engineers from 39 institutions. The detector is located at the Sanford Underground Research Facility in South Dakota, a former gold mine, buried about 1.6 kilometers beneath the rock. It is buried so deep to shield against cosmic rays. On the ground, a large number of particles bombard the earth every moment, which would create a lot of false signals in the detector. A mile of rock is equivalent to a huge blanket, blocking out most of the interference.
The detector's core is a large cylindrical vessel filled with ultra-pure liquid xenon. Ten tons of xenon are cooled below minus 108 degrees Celsius to form a liquid. Hundreds of photomultiplier tubes line the vessel walls, watching over the interior like so many eyes. Any particle entering the vessel that collides with a xenon atom produces faint light and charge signals, which these eyes capture and record. Should a dark matter particle—if it exists—occasionally collide with a xenon nucleus, it generates two signals: a faint flash of ultraviolet light, and charge released as the struck nucleus recoils forward.
This is not an ordinary flash. Researchers analyzed 220 days of data from March 2023 to April 2024 and discovered an anomaly, carefully ruling out all known background sources: natural radioactivity, cosmic ray remnants, and minute radiation from the detector materials themselves... none of which could explain the flash.
The recoil energy of this signal is as high as 248 kiloelectronvolts, far exceeding the low-energy region below 50 kiloelectronvolts that has been the focus of previous searches. If it is indeed a signal from dark matter, the mass of the weakly interacting massive particle that produced this signal, a major dark matter candidate, would be at least 200 times that of a proton.
What's even more puzzling is its location, as it appears in a region where researchers expected to see a dark matter signal with very low background interference, and its behavior is completely unlike any known background signal, which means scientists may have actually discovered something new.
Furthermore, its statistical significance reached 2.6 sigma. What does this mean? This number means that the event still has a 1 in 200 chance of being explained by known background processes, in other words, the probability that it comes from some unknown source is as high as 99.5%.
However, in particle physics, a 5-sigma significance must be reached, meaning the probability that the signal is merely a coincidence must be less than 0.00003%, for the discovery to be conclusive.
The experiment's spokesperson said the signal is either an extremely rare background event or the first hint of dark matter, though they also emphasized that they have simply found something interesting and are seeking the opinion of scientists, without claiming it is definitely dark matter.
Dark Matter: The Universe's Greatest Enigma
When you gaze up at the stars and see the stars, planets, and nebulae, all the things that can be observed with a telescope, they account for only about 15% of the total matter in the universe. The remaining 85% is invisible, non-luminous, and completely invisible matter. Scientists call it dark matter.
Though invisible, dark matter reveals its presence through gravitational effects. Galaxies spin far faster than they should—based on the mass of their visible matter alone, they ought to have flown apart long ago. Yet they haven't, which means some unseen extra mass must be holding them together. That invisible cosmic glue is dark matter.
For the past 40 years, physicists have been searching for the true identity of dark matter. The most mainstream hypothesis is that dark matter is composed of a type of particle called a weakly interacting massive particle, or WIMP. This particle is much heavier than a proton and rarely interacts with ordinary matter, passing through everything like a ghost, but extremely occasionally, it collides with an atomic nucleus, leaving a faint flash.
This is what the LZ detector is capturing.
Why So Sinister
The paradox of this signal is not only that it is hard to explain based on known backgrounds, but also that it appeared in a way that is not right.
According to most standard theories, if LZ is to observe a high-energy weakly interacting massive particle event, it should have captured tens or even hundreds of low-energy events first. This is like a person searching for mushrooms in a forest - if they really find a large mushroom, logically there should be many small mushrooms around it.
But LZ detected no anomalous signals from any low-energy events. Only one high-energy event stood there in isolation. Researchers racked their brains, trying to explain it away as other known background events—to no avail.
In other words, if this signal is indeed dark matter, the true nature of dark matter may be far more complex than physicists have thought.
"Given that we still don't know what dark matter is, it's likely to be something beyond our expectations, so I don't find it surprising," said a member of the experimental collaboration.
Reaction from the Physics Community
The global physics community was quickly abuzz after the news was announced. Nature magazine wrote in its report: "Just one data point, yet enough to send shivers down the spine of physicists."
"I've been waiting for a positive outcome, my goodness, for 40 years," said Katherine Freese, a theoretical physicist at the University of Texas at Austin, whose early research provided crucial inspiration for the search for dark matter. "I can assure you, I am extremely, extremely excited about the results from LZ."
Eric Dahl from Northwestern University has been working in the field of dark matter detection for 20 years. "This is the most interesting event I've ever seen," he said. "Over the past year, we've spent a lot of time trying to find an explanation for this event. After crunching the numbers, we can't find any known source that could produce this kind of signal with more than a 1% probability."
But some people are also taking a cautious stance.
"This is an incident worth investigating," said Professor Liu Jianglai, a leading figure in China's dark matter research field at Shanghai Jiao Tong University. "It's indeed eye-catching and will undoubtedly stir up excitement in the field."
Juan Collar of the University of Chicago, who is not a member of the LZ collaboration, praised the result, saying the LZ collaboration has done a good job of ruling out radioactive background interference. However, he also pointed out a key issue: regardless of whether the cause is radioactive background or dark matter, it should also bring a large number of low-energy recoil events, which LZ did not observe. "This is interesting," Collar said, "but also puzzling."
What Does the Future Hold?
The LZ collaboration team has currently submitted relevant papers, and is collecting more data, with over 700 days of blind data awaiting analysis. The so-called blind data refers to the fact that before analysis is completed, researchers do not know whether the data contains signals, which is done to avoid subjective bias.
"This is 200 days' worth of data, and in fact, we have more than 700 days' worth of blinded data in hand, which puts us in a favorable position," the spokesperson for the experiment said.
If more similar events appear in subsequent data, it will greatly increase the likelihood of dark matter. At that time, two other competing experimental groups, PandaX-4T from China's Jinping Underground Laboratory and XENONnT from Italy's Gran Sasso National Laboratory, will also assist in verifying LZ's discovery.
If no similar cases emerge in subsequent data, this signal may simply be an extremely rare background event—statistical noise.
Time Running Out: The Last Window for Dark Matter Search
This detection also has a special historical context.
As detectors become increasingly large and sensitive, they will soon be overwhelmed by signals from another ghostly particle, neutrinos. Originating from the sun and the Earth's atmosphere, the sheer volume of neutrinos will flood detectors like noise, making the search for dark matter extremely difficult.
"This is the last window of opportunity, and if you haven't found the weakly interacting massive particles by then, you'd better shut down at some point in the next decade," Collar said.
"This signal is likely to be the beginning of an extraordinary journey."
Regardless of the outcome, the story is already compelling enough. After 40 years of coming up empty-handed, dark matter hunters have finally seen a signal that cannot be ignored. If they are unlucky, this eerie signal may turn out to be a false alarm, but if they are lucky, it could be humanity's first real touch with the most mysterious substance in the universe.
In the coming months, physicists around the world will be closely watching the data to be released by LZ. This is the closest to an answer in 40 years, and it is the last window of opportunity. If it's not a false alarm, human understanding of the universe will usher in another revolutionary leap.
References
Scientists have made a potential breakthrough in the search for dark matter, a mysterious substance that is believed to make up approximately 27% of the universe's mass-energy density, according to a recent study published in the journal Nature. Researchers at the XENON1T experiment, located at the Gran Sasso National Laboratory in Italy, claim to have detected a signal that could be indicative of dark matter particles interacting with a tank of super-cooled xenon. The XENON1T experiment uses a tank of xenon, a noble gas, to detect potential dark matter particles, which are thought to interact with normal matter only through the weak nuclear force and gravity. The signal detected by the XENON1T experiment is a small excess of events that cannot be explained by known background sources, and it has a statistical significance of 3.0 sigma, which means that there is only a 1 in 740 chance that the signal is due to a statistical fluctuation. However, the researchers caution that the signal is still preliminary and requires further verification. If confirmed, this discovery could be a major breakthrough in the search for dark matter, which has been ongoing for decades. The XENON1T experiment is one of several experiments around the world that are searching for