Hunting the Universe's Darkest Secret
The mystery of dark matter is one of science's greatest puzzles, but new technologies are bringing us closer than ever to a solution.
Dark matter constitutes roughly 85% of the matter in the universe, yet it has never been directly observed. It does not emit, absorb, or reflect light, making it completely invisible to our telescopes. Its presence is known only through its gravitational pull on stars and galaxies. For decades, scientists have been searching for this elusive substance with increasingly sophisticated experiments. Today, the field is undergoing a dramatic transformation, moving from a singular focus on one type of particle to a broad hunt across dozens of possible candidates, employing everything from vats of liquid xenon buried deep underground to novel quantum sensors and materials that function as "cosmic car radios."
The search for dark matter has entered a new era. For forty years, the leading candidate has been a hypothetical particle known as a WIMP (Weakly Interacting Massive Particle). These particles, if they exist, would have a mass comparable to that of an atomic nucleus and would interact with normal matter only through the weak nuclear force and gravity. Decades of effort have been poured into building ever-more-sensitive detectors to find them8 .
The landscape is now shifting. As reported at the International Cosmic Ray Conference (ICRC) 2025, the field is in a period of significant transition. While WIMP searches continue, non-WIMP candidates—such as axions, sub-GeV particles, and primordial black holes—are moving from the periphery to the center of attention1 . In fact, for the first time in recent history, a major physics conference devoted more plenary space to these alternative candidates than to canonical WIMPs1 . This reflects a strategic broadening of the hunt, driven by the tenacity of the mystery.
Traditional approach using large detectors to capture collisions between WIMPs and atomic nuclei.
New methods sensitive to lighter particles that interact with electrons rather than nuclei.
The relentless null results from WIMP detectors have prompted physicists to reconsider their assumptions. If WIMPs were the size of a bowling ball, trying to detect them by having them bump into the atomic nucleus of a xenon atom (another bowling ball) was a sensible strategy. But what if dark matter is more like a swarm of ping-pong balls? A ping-pong ball colliding with a bowling ball would not make it budge, and the signal would go completely unnoticed8 .
This realization has spurred the development of technologies sensitive to much lighter and "wimpier" particles that could interact with the tiny electrons orbiting an atom, rather than with the massive nucleus at its center8 .
One of the world's most sensitive WIMP hunters is the LUX-ZEPLIN (LZ) experiment, a masterpiece of modern engineering operating nearly a mile underground at the Sanford Underground Research Facility in South Dakota7 . Its mission is to capture the faint, tell-tale jolt of energy from a WIMP colliding with the nucleus of a xenon atom.
The core of the LZ detector is a two-story-tall titanium tank filled with 10 tonnes of ultrapure liquid xenon, chosen for its density and its ability to produce two signals—a flash of light and a cloud of electrons—when a particle interacts with it7 . The entire apparatus is built like a protective onion to shield the sensitive xenon from any distracting background "noise."
The experiment is housed deep underground, where the rock overhead shields it from cosmic rays from space7 .
The central xenon vessel is surrounded by a huge tank filled with a gadolinium-loaded liquid scintillator. This layer is designed to catch neutrons, subatomic particles that can mimic a WIMP signal7 .
The detector is constructed from thousands of parts specially manufactured to have extremely low levels of natural radioactivity. It is also further shielded with ancient, low-radiation lead and specially grown copper8 .
Operating the LZ detector is like trying to hear someone whisper in a stadium full of people8 . The process is meticulous:
The detector runs continuously, with its sensitive xenon core maintained in a perfectly stable, ultra-clean, and dark state.
If a particle enters the xenon and collides with a nucleus, two things happen: a prompt flash of light and the release of electrons.
The initial light pulse is picked up by sensors. An electric field drifts electrons to produce a second flash of light.
Background signals are identified and rejected using the outer detector layers and sophisticated software.
To prevent unconscious bias, the LZ collaboration even uses a technique called "salting," where fake WIMP signals are secretly added to the data during collection. Researchers don't know the true data until the very end of the analysis, ensuring their results are completely objective7 .
The LZ experiment, after analyzing 280 days of data, has not yet found a definitive WIMP signal. However, its results are profoundly important. By setting ever-more-stringent limits on what dark matter is not, LZ is narrowing the field of possibilities, allowing physicists worldwide to refine their theories and focus their searches7 . The experiment will continue to collect data until 2028, pushing further into uncharted territory.
The search for dark matter now encompasses a wide range of theoretical particles, each requiring different detection strategies.
Theoretical Mass Range: Ultralight (μeV–meV)
Detection Method: Conversion to photons in strong magnetic fields, or using specialized materials6
Theoretical Mass Range: Across many orders of magnitude
Detection Method: Gravitational lensing and other astrophysical observations1
| Background Particle | Why it Mimics a WIMP | LZ's Veto Method |
|---|---|---|
| Neutrons | Causes a nuclear recoil, identical to a WIMP's expected signal. | The Outer Detector (OD) sees a coincident signal, allowing the event to be tagged and rejected7 . |
| Radon | Undergoes a sequence of decays that can be mistaken for a WIMP. | Advanced analysis identifies the full decay chain, marking it as a background event7 . |
| Gamma Rays & Beta Particles | Can cause electron recoils in the xenon. | The ratio of the two light signals (S1 and S2) is different for electron recoils vs. nuclear recoils, allowing for statistical subtraction7 . |
The diverse hunt for dark matter requires an equally diverse arsenal of tools and materials. The following details some of the essential components powering these cosmic searches.
The target material in detectors like LZ and XENONnT. Its dense nuclei are a target for WIMP collisions, and its scintillation and ionization properties allow for precise particle identification7 .
A liquid placed in the outer veto system of detectors. It captures neutrons (a key background) and produces light, signaling their presence so an event can be rejected7 .
Highly sensitive sensors capable of counting single electrons. This allows them to search for lightweight dark matter that would interact with an atom's electrons, not its nucleus8 .
Wires cooled to superconducting temperatures that can detect single, low-energy photons. This makes them ideal for hunting for very lightweight dark matter particles5 .
A quantum material engineered to host "axion quasiparticles." It could act as a dark matter detector by reacting when a true axion particle strikes it6 .
The next decade promises to be a revolutionary period for dark matter research. Several new projects and technologies are poised to take the hunt to the next level.
Plans are already underway for an even larger successor to LZ, tentatively called XLZD7 . The DarkSide-20k experiment, currently under construction in Italy, will use 20 tonnes of liquid argon to continue the search for WIMPs with even greater sensitivity1 .
Researchers at SLAC and elsewhere are exploring the use of quantum devices as dark matter sensors. These devices are so sensitive that they could potentially detect "thermalized dark matter"—particles that have been trapped by Earth's gravity for eons and move much more slowly than galactic dark matter.
The Cherenkov Telescope Array Observatory (CTAO), under construction in Chile and Spain, will begin observations in 2027. It will image gamma rays with unprecedented resolution, potentially confirming whether the mysterious glow at the Milky Way's center is indeed the signature of annihilating WIMPs4 .
Theoretical physicists continue to propose novel candidates, such as superheavy gravitinos. These charged particles would be detectable through a faint "glow" they produce as they travel through the detection fluid in giant observatories like JUNO9 .
The search for dark matter is more dynamic and innovative than ever. While the mystery persists, the scientific community's strategy—broad, collaborative, and technologically brilliant—ensures that we are steadily closing in on one of nature's best-kept secrets. The question is no longer if we will find dark matter, but what it will turn out to be, and how its discovery will reshape our understanding of the cosmos.