Axions were first proposed to solve the strong CP problem in quantum chromodynamics (QCD). In simple terms, QCD, the theory describing the strong nuclear force, should allow for a strange kind of symmetry violation (charge-parity violation) in the interactions of quarks and gluons. But in reality we don’t see this happening, and nobody knows why. Axions are one way out: these hypothetical particles could naturally “cancel out” the effect, explaining why the strong interaction behaves the way it does without breaking symmetry where it shouldn’t.
Axions might be more than a neat mathematical fix though. They could also be the long-sought answer to dark matter. Unlike ordinary matter, dark matter doesn’t interact via electromagnetism, meaning it doesn’t emit, absorb, or reflect light. It only reveals itself through gravity, shaping galaxies and cosmic structures. Axions, if they exist, would be extremely light, barely interacting with other particles, and could form an enormous, invisible field filling the universe, precisely the kind of thing dark matter needs to be.
Unlike most particles in the Standard Model, axions are predicted to be neutral, extremely light and incredibly weakly interacting. This makes them almost impossible to detect directly, but researchers are trying. Experiments like ADMX (Axion Dark Matter Experiment) are searching for axions by looking for their potential interactions with electromagnetic fields, hoping to finally catch one.