Axions are hypothetical subatomic particles that are very light and weakly interacting with normal matter. They are predicted by extensions of quantum field theory, specifically extensions of the Standard Model of particle physics. Axions are proposed to explain the absence of the so-called strong CP problem, which is a theoretical inconsistency of the Standard Model.
Axions are predicted to have a mass of around 10^-5 eV/c^2, which is about 10 billion times smaller than the mass of an electron. They are also predicted to be very weakly interacting with normal matter, with a cross-section of around 10^-55 cm^2. This means that they are very difficult to detect, and only a few experiments have been able to set limits on their existence.
If axions do exist, they could have a number of important implications for physics and cosmology. For example, they could be a component of dark matter, the mysterious form of matter that makes up about 27% of the universe. Axions could also be used to explain the existence of pulsars, which are rapidly rotating neutron stars.
The search for axions is an ongoing effort, and a number of experiments are currently underway. If axions are ever detected, it would be a major breakthrough in physics, and would provide important insights into the nature of the universe.