The Hunt for Dark Matter: Axions vs. WIMPs Explained

For decades, the search for the invisible substance that makes up roughly 85% of the matter in our universe has focused on one primary suspect. Now, with those searches coming up empty, physicists are pivoting. The hunt is moving away from heavy particles toward something much lighter and stranger. Here is why the scientific community is shifting its bets from WIMPs to axions.

The Reign of the WIMP

For over thirty years, the leading candidate for dark matter was the Weakly Interacting Massive Particle, or WIMP. This theory was popular because it fit perfectly with a concept called Supersymmetry. Supersymmetry suggests that every known particle in the Standard Model (like electrons and quarks) has a heavier, invisible partner.

Scientists liked WIMPs because they theoretically had a mass ranging from 10 to 1,000 times that of a proton. If these particles existed, they would have been produced in the Big Bang in just the right amounts to account for the dark matter we see holding galaxies together today. This coincidence was so elegant that physicists called it the “WIMP Miracle.”

However, the miracle has faded. The world’s most sensitive detectors have spent years waiting for a WIMP to crash into an atomic nucleus, yet they have seen nothing.

The Major WIMP Experiments

To understand why confidence is waning, you have to look at the scale of the experiments that have failed to find them.

• LUX-ZEPLIN (LZ): Located nearly a mile underground in the Sanford Underground Research Facility in South Dakota, this experiment uses 10 tons of liquid xenon. The idea is that if a WIMP hits a xenon atom, it should produce a tiny flash of light. As of its latest major data releases in 2023 and 2024, LZ has found no trace of WIMPs.
• XENONnT: Operating deep under a mountain in Italy at the Gran Sasso National Laboratory, this detector is similar to LZ. It recently ruled out a significant range of potential WIMP masses, further shrinking the hiding places for these particles.
• PandaX-4T: Based in the China Jinping Underground Laboratory, this experiment has also reported null results, confirming the findings of its Western counterparts.

Because these massive, expensive projects have not found a single signal, many physicists believe it is time to look elsewhere.

Enter the Axion

As the window for WIMPs closes, the door has opened for a very different candidate: the axion. Unlike the heavy WIMP, an axion is theorized to be ultra-light. In fact, an axion could be trillions of times lighter than an electron.

The axion was not originally proposed to explain dark matter. In 1977, physicists Roberto Peccei and Helen Quinn proposed the particle to solve a completely different issue in quantum physics known as the “Strong CP Problem.” This problem questions why the strong nuclear force affects particles and their antimatter partners equally, even though the math suggests it shouldn’t.

If axions exist to solve that problem, they would also have the exact properties needed to be dark matter. They would be abundant, cold, and interact very weakly with regular matter.

How Axions Differ from WIMPs

The best way to understand the difference is to imagine how we try to detect them.

• WIMPs are like bowling balls: We expect them to smash into atomic nuclei like a bowling ball hitting pins. We listen for the “clack” of the impact.
• Axions are like radio waves: Because they are so light, they act more like waves than particles. We cannot wait for them to hit something. Instead, we have to “tune” a detector to their specific frequency to hear them.

The New Tools of the Hunt

Because axions behave differently than WIMPs, scientists need entirely different machinery to find them. The current strategy relies on a piece of physics called the Primakoff effect. This theory states that in the presence of a very strong magnetic field, an axion can transform into a photon (a particle of light).

Basically, if you build a powerful magnetic chamber and shield it from all outside interference, an axion passing through should spontaneously turn into a microwave signal.

Leading Axion Experiments

The shift in focus is visible in the funding and construction of new “haloscopes” (detectors designed to find the dark matter halo).

• ADMX (Axion Dark Matter eXperiment): Located at the University of Washington, ADMX is the flagship experiment for axion detection. It uses a large superconducting magnet and a resonant cavity. The team slowly scans through different frequencies, much like turning the dial on an old radio, hoping to catch a broadcast from dark matter. ADMX is currently the only experiment sensitive enough to detect faint axions in the most likely mass ranges.
• HAYSTAC: Housed at Yale University, the Haloscope at Yale Sensitive to Axion Cold Dark Matter (HAYSTAC) acts as a testbed for new technologies. It focuses on higher frequencies than ADMX.
• CERN’s CAST and BabyIAXO: At CERN in Europe, researchers look for axions coming specifically from the Sun. The CAST experiment points a magnet at the Sun to catch solar axions. Its successor, BabyIAXO, is currently in development and will be much larger and more sensitive.

Why the Shift Matters

The transition from WIMPs to axions represents a major moment in modern physics. If WIMPs are truly ruled out, it casts doubt on Supersymmetry, which has been a guiding principle for particle physics for decades.

Conversely, if axions are found, it hits two birds with one stone. It would finally identify dark matter, resolving the mystery of invisible mass in the universe. Simultaneously, it would validate the Peccei-Quinn theory, fixing a 40-year-old gap in our understanding of nuclear forces.

This search requires extreme precision. The signals scientists are looking for are incredibly faint—equivalent to detecting a single cell phone signal on Mars from Earth. To achieve this, experiments like DMRadio are now incorporating quantum sensors to reduce background noise to fundamental limits.

While the WIMP has not been completely abandoned, the excitement is clearly with the axion. The next decade of physics will likely be defined by tuning our cosmic radios, listening for the whisper of the invisible universe.

Frequently Asked Questions

What happens if we never find WIMPs or Axions? If both candidates are ruled out, physicists will have to reconsider gravity itself. Theories like MOND (Modified Newtonian Dynamics) suggest that perhaps there is no invisible matter, but rather that our understanding of gravity is flawed at large scales. However, current data supports the existence of matter over modified gravity.

Are there other candidates besides these two? Yes. While WIMPs and axions are the leaders, there are other theories. These include sterile neutrinos (heavier cousins of regular neutrinos) and “fuzzy” dark matter, which consists of particles even lighter than axions that behave almost entirely like waves.

Can dark matter hurt us? No. Dark matter passes through regular matter constantly without interacting. Billions of dark matter particles are likely passing through your body right now, but because they do not interact with electromagnetism, they have no effect on your biology.

How do we know dark matter exists if we can’t see it? We see its gravitational effects. When we look at distant galaxies, they spin much faster than they should based on the visible stars we can see. Without extra invisible mass holding them together, these galaxies would fly apart. We also see light bending around massive invisible objects, a phenomenon known as gravitational lensing.