Dark matter makes up 27% of the universe but hasn't been found yet. Which quantum candidates — WIMPs, axions, neutrinos — could it be?
🌌 The Mystery of Dark Matter
In 1933, Swiss astronomer Fritz Zwicky studied the Coma galaxy cluster and observed something disturbing: the galaxies were moving so fast that their visible mass was insufficient to hold them together. Something invisible — which he called "dunkle Materie" (dark matter) — had to provide the extra gravitational pull.
Forty years later, American astronomer Vera Rubin confirmed the mystery. Studying galaxy rotation curves in the 1970s, she discovered that stars at the edges of galaxies were moving just as fast as those near the center — contrary to what Newton's law of gravity predicted. The only explanation: a vast, invisible dark matter halo surrounded every galaxy.
Today, gravitational lensing — the bending of light around large masses, as predicted by Einstein — confirms that there is far more mass in the universe than we can see. According to measurements by the Planck satellite, dark matter constitutes approximately 27% of the total energy content of the universe, while ordinary (baryonic) matter accounts for only 5%. The remaining 68% is the even more mysterious dark energy.
🔬 WIMPs: The Most Popular Candidates
For decades, WIMPs (Weakly Interacting Massive Particles) have been the leading candidates. These hypothetical particles have a mass between 10 and 1,000 GeV (like heavy atomic particles) and interact only via the weak nuclear force and gravity.
The beauty of WIMPs lies in the so-called "WIMP miracle": if a particle with such mass and weak interaction was created in the early universe, its present-day abundance would match exactly the observed density of dark matter. Supersymmetry (SUSY) naturally predicts such particles, with the most famous being the neutralino — the lightest supersymmetric partner.
Experiments like XENON1T and its successor XENONnT at Gran Sasso in Italy, as well as LUX-ZEPLIN (LZ) in the USA, search for WIMPs using tons of liquid xenon. The idea is simple: if a WIMP strikes a xenon nucleus, it will produce a tiny signal of light and charge. So far, no WIMP has been detected — but each new experiment increasingly constrains the parameter space.
⚛️ Axions: The Lightweight Candidate
Axions were originally proposed by Roberto Peccei and Helen Quinn in 1977 to solve an entirely different problem: why doesn't the strong nuclear force violate CP (charge-parity) symmetry? Their solution — the Peccei-Quinn mechanism — inevitably predicts a new, extremely light particle.
Axions are incredibly light: their mass is estimated between 10⁻⁶ and 10⁻³ eV, or trillions of times lighter than the electron. If they constitute dark matter, they must exist in enormous quantities — perhaps forming a Bose-Einstein condensate on cosmological scales.
The ADMX (Axion Dark Matter eXperiment) at the University of Washington uses strong magnetic fields and extremely sensitive microwave detectors to search for the conversion of axions into photons — the so-called Primakoff effect. Technology is improving rapidly, and axions are now considered equally likely candidates as WIMPs.
👻 Sterile Neutrinos and Other Candidates
Sterile neutrinos represent yet another proposal. Unlike ordinary neutrinos that interact via the weak force, sterile neutrinos don't interact with any force except gravity. If they exist, they could have a mass on the keV scale, making them candidates for “warm” or “tepid” dark matter.
A more exotic proposal involves primordial black holes: microscopic black holes that formed in the first fractions of a second after the Big Bang. Stephen Hawking and Bernard Carr proposed this idea in the 1970s, and while gravitational lensing has ruled out certain mass ranges, some windows remain open.
Finally, "fuzzy" dark matter consists of ultralight bosons with mass ~10⁻²² eV. The de Broglie wavelength of these particles reaches kiloparsec scales, creating quantum interference patterns on galactic scales — a scenario that could explain why dwarf galaxies are fewer than the standard model predicts.
🏗️ Detection Experiments
The search for dark matter proceeds on three fronts. Direct detection looks for signals from dark matter particles colliding with atomic nuclei in underground laboratories, far from cosmic radiation. The most important are located at Gran Sasso (Italy), SNOLAB (Canada), and the Sanford Underground Research Facility (USA).
Indirect detection searches for annihilation products: if two dark matter particles meet and annihilate, they would produce gamma rays, neutrinos, or antiparticles. Telescopes like Fermi-LAT and detectors such as IceCube in Antarctica search for such signals from the center of our galaxy or from the Sun.
Finally, the Large Hadron Collider (LHC) at CERN searches for dark matter particles through "missing energy": if a dark matter particle is produced in a proton collision, it will leave the detector without a trace, appearing as an energy imbalance. To date, no such imbalance has been observed beyond the predictions of the Standard Model.
💡 Alternative Theories
What if we don't need dark matter at all? Some physicists argue that the problem lies in our understanding of gravity, not in the existence of invisible matter.
MOND (Modified Newtonian Dynamics), proposed by Mordehai Milgrom in 1983, modifies the law of gravity at very low accelerations. It explains galaxy rotation curves remarkably well without dark matter, but faces serious problems at the scale of galaxy clusters.
Dutch physicist Erik Verlinde proposed another radical alternative: emergent gravity. According to this theory, gravity is not a fundamental force but emerges from entropic interactions, and what we call “dark matter” is a phenomenon of this emergent dynamics.
However, the Bullet Cluster — two galaxy clusters that collided — provides strong evidence in favor of dark matter. During the collision, the hot gas (visible matter) slowed down, but the gravitational mass (measured through gravitational lensing) passed through without deceleration, exactly as one would expect from particles that don't interact electromagnetically. This observation is considered by many as the most convincing proof that dark matter is real matter and not modified gravity.