The Invisible Majority
A single flash, a mile underground
On 16 June 2023, deep beneath the Black Hills of South Dakota, ten tonnes of ultrapure liquid xenon registered one tiny flash of light. Three years later, in September 2026, scientists told the world they still cannot explain it with anything known.
The flash came from LUX-ZEPLIN (LZ), the world’s most sensitive dark matter detector. Presenting the analysis at the TeV Particle Astrophysics conference in Japan on 1 September, the collaboration reported a signal at 2.6 sigma, roughly a one-in-200 chance of being a background fluke.
That falls far short of the 5-sigma bar physics demands for a discovery, about one chance in 3.5 million. LZ’s own spokesperson, Rick Gaitskell of Brown University, was careful: the team is not claiming dark matter. Yet for a field that has searched for four decades and found nothing, one unexplained event in the right place is enough to make the world lean in.
This is the story of that search: the invisible substance that outweighs everything we can see by more than five to one, and why finding it may be the defining scientific achievement of this century.
The universe we cannot see
Ordinary matter, everything made of atoms, is only about 5 percent of the cosmos. Dark matter makes up roughly 27 percent, and dark energy the remaining 68 percent. Put simply, the stars, planets and people we know are a rounding error.
Dark matter does not shine, absorb or reflect light. We know it exists only because of its gravity, and the evidence has piled up from several independent directions.
- Galaxy clusters (1933). Swiss astronomer Fritz Zwicky found galaxies in the Coma Cluster moving so fast that visible mass could not hold them together. He called the missing mass “dunkle Materie”.
- Spinning galaxies (1970s). Vera Rubin and Kent Ford showed that stars at the edges of spiral galaxies orbit as fast as those near the centre. Only an unseen halo of mass could explain it.
- Gravitational lensing. Mass bends light. Maps of how background galaxies are distorted reveal far more mass than the visible matter can supply.
- The Bullet Cluster (2006). In two colliding clusters, the hot gas slowed down while most of the mass, traced by lensing, sailed straight through. It was the clearest sign that dark matter is a real substance, not a quirk of gravity.
- The cosmic microwave background. The Big Bang’s afterglow, mapped by WMAP and Planck, carries a pattern that only fits if dark matter outweighs ordinary matter about five to one.
Without dark matter, galaxies would not have formed in the time available. It is the invisible scaffolding on which the visible universe was built.
The suspects
Nobody knows what dark matter is made of. Physicists have a line-up of suspects, each implying different physics and each demanding a different kind of search.
| Candidate | What it is | How scientists hunt it |
|---|---|---|
| WIMPs | Heavy particles that barely interact with ordinary matter | Underground xenon and argon detectors; the Large Hadron Collider |
| Axions | Ultra-light particles first proposed to fix a puzzle in nuclear physics | Tuned microwave cavities in strong magnetic fields (e.g. ADMX) |
| Sterile neutrinos | A heavier, even shyer cousin of the known neutrinos | X-ray telescopes looking for faint decay lines |
| Primordial black holes | Black holes formed in the first second after the Big Bang | Microlensing surveys; gravitational-wave detectors |
| Modified gravity (MOND) | No new matter; gravity itself behaves differently at low accelerations | Galaxy rotation data; struggles to explain clusters and the Bullet Cluster |
For thirty years the WIMP was the favourite. It emerged naturally from theories of particle physics, and calculations showed it would leave behind almost exactly the amount of dark matter we observe, a coincidence physicists called the “WIMP miracle”.
Decades of null results have dented that confidence and pushed attention towards axions and more exotic ideas. That is exactly why a possible WIMP-like event from LZ has drawn so much attention.
The hunt: underground, in colliders and across the sky
The search runs on three fronts. Direct detection waits for a dark matter particle to bump into an atom. Collider searches try to create one. Indirect and astronomical searches look for its fingerprints across the cosmos.
How LZ works
LZ sits nearly a mile below ground at the Sanford Underground Research Facility, where rock shields it from cosmic rays. When a particle strikes a xenon nucleus, it gives off a first flash of light and frees electrons. An electric field pulls those electrons to the top of the tank, where they make a second flash. The pair of flashes tells scientists the energy and position of each hit.
What was seen
The new analysis covered 220 live days of data from March 2023 to April 2024. Earlier searches of the same data had looked at low energies. This time the team widened the window to higher-energy collisions, which some WIMP models predict.
They found one event, a nuclear recoil of about 248 keV, in a region where known backgrounds are expected to be very low. Lead author Sam Eriksen of the University of Bristol said the team spent months ruling out ordinary explanations. The paper has been submitted to Physical Review Letters.
What it does and does not mean
One event cannot establish a discovery. Rare background events, statistical chance or an unknown detector effect remain possible. The test is simple: LZ is collecting data towards a total of 1,000 live days. If the signal is real, more such events should appear; if not, it will fade.
The wider net
- Rival detectors. XENONnT in Italy and PandaX-4T in China use similar xenon technology and can check LZ’s result independently.
- The next generation. The proposed XLZD observatory would scale liquid xenon up several-fold.
- Telescopes. Europe’s Euclid mission and the Vera C. Rubin Observatory in Chile are mapping dark matter’s distribution across billions of galaxies through lensing.
- Colliders. CERN’s Large Hadron Collider continues to search for missing energy that could betray new invisible particles.
Cracks in the standard picture
The reigning model of cosmology, known as Lambda-CDM, assumes dark matter is “cold”: slow-moving particles that clump easily into galaxies. It has been spectacularly successful, but it is no longer unquestioned.
In January 2026, researchers at the University of Minnesota argued that dark matter may have been born hot, racing near the speed of light just after the Big Bang. By studying a turbulent early phase called post-inflationary reheating, they showed such particles could cool in time to build galaxies. If correct, it widens the range of particles that could be dark matter.
Other puzzles persist. Dwarf galaxies look less densely packed at their centres than simulations predict. Astronomers also disagree on how fast the universe is expanding, the so-called Hubble tension. Some see these as clues to new physics in the dark sector; others expect better data to resolve them.
The honest position is that dark matter’s existence is supported by overwhelming evidence, while its nature remains entirely open.
