Researchers analyzing a decade of geomagnetic data from the British Geological Survey’s Eskdalemuir Observatory have identified unexplained signal candidates that could point to ultralight dark matter. At the same time, deep underground in South Dakota, the LUX-ZEPLIN experiment recorded an unusual particle interaction with a 0.5 percent chance of being a fluke.
Dark matter remains one of the biggest unsolved mysteries in modern physics. Astronomers estimate that it accounts for about a quarter of the universe’s total energy content, with ordinary matter making up about 5% and dark matter comprising roughly 85% of all matter in the universe. Because it does not emit, absorb, or reflect light, scientists cannot observe it directly. Instead, its presence is inferred through its gravitational effects on galaxies and galaxy clusters.
Using Earth Itself as a Planetary Dark Matter Detector
Traditional axion experiments rely on powerful laboratory magnets to convert hypothetical dark matter particles into detectable photons. However, physical space limits those systems to a relatively small area. To bypass this scale limitation, researchers from Kyoto University, Hiroshima University, and Nihon University turned their attention to the entire planet. The Earth-ionosphere cavity acts as a natural resonator that amplifies electromagnetic waves in the exact mass range the team wanted to probe.
The space between Earth’s surface and the electrically conductive ionosphere traps and strengthens electromagnetic waves at specific frequencies. While previous theoretical models could only describe frequencies below 1 Hz, the research team developed a new framework incorporating atmospheric electrical conductivity. Their calculations showed that the Earth-ionosphere cavity can amplify signals near 8 Hz, allowing reliable predictions up to about 30 Hz.
Ten Years of Eskdalemuir Observatory Data Put to the Test
To test this planetary-scale framework, the investigators examined geomagnetic observations recorded from 2012 through 2022 at the British Geological Survey’s Eskdalemuir Observatory. After stripping away artificial human-made noise, they searched for persistent signals concentrated within a very narrow frequency range.
The mathematical model predicted distinct signatures for the two leading dark matter candidates. Axion signals should vary depending on location because they require Earth’s magnetic field to generate electromagnetic waves, with the strongest sensitivity expected in Southeast Asia. Dark photon signals, by contrast, should appear at nearly identical strengths across the globe because they do not require an external magnetic field to produce the same effect.
The analysis yielded no confirmed axion discovery, but the absence of a signal allowed researchers to place new constraints on how readily axions interact with light. These ground-based limits were roughly 100 times tighter than previous best results from terrestrial experiments. Meanwhile, the dark photon search produced several intriguing signal candidates. The exact origin of these signals remains unknown, leaving the true identity of dark matter unresolved.
A Deep Underground Clue in South Dakota
While atmospheric researchers scanned geomagnetic records, a separate international collaboration sought answers nearly a mile underground. Managed by the US Department of Energy’s Lawrence Berkeley National Laboratory, the LZ Dark Matter Experiment brings together 250 scientists and engineers from 39 institutions worldwide. The detector operates inside a former gold mine at the Sanford Underground Research Facility in Lead, South Dakota, where surrounding bedrock shields it from radiation arriving from space.
The main detector houses 10 tonnes of ultrapure liquid xenon cooled below -108 degrees Celsius (-164.2 degrees Fahrenheit). When a particle strikes a xenon atom, it produces two distinct flashes of light that researchers measure to study the interaction. On June 16, 2023, the detector recorded a single particle interaction that deposited far more energy than expected from a standard weakly interacting massive particle, or WIMP.
Analysts calculate there is only a 0.5 percent chance that the signal came from a known source, representing a roughly 2.6-sigma result. Physics standards require a 5-sigma confidence level before claiming a definitive discovery.
Evaluating Background Signals and Next Steps
Scientists cannot yet rule out ordinary background sources for the South Dakota event. Radioactive decay within detector tank materials or within the xenon itself remains a persistent variable.
Both the atmospheric framework tested on British Geological Survey data and the deep-underground findings from South Dakota highlight the growing complexity of modern physics research.
