Physicists often ponder in an unusual way: they use one unsolved problem as a hypothetic solution to another, in the hope of resolving both.
The strong CP problem is the perfect example.
In the standard model of physics, a partially proven theory that describes our world at the smallest scale, scientists recognize the mirrored existence (symmetry) of particles in opposite charge conjugations (C) and parity (P).
The C- and P-symmetry can be broken, in some cases simultaneously, except in one scenario. For instance, the symmetry between matter and antimatter (C-symmetry) can fall apart in weak nuclear interactions, but not in strong interactions, even though nothing stops that from happening in theory. Thus, the conservation of CP symmetry is considered a "fine-tuning" issue, a puzzling topic scientists have been trying to address in order to optimize or "fix" the standard model.
Enter problem number two: dark matter is a kind of bizarre entity that makes up approximately 85% of the mass in our universe. But due to its lack of interaction with light or any electromagnetic radiation, scientists have not been able to detect them.
In the late 1970s, physicists Roberto Peccei and Helen Quinn proposed an elegant mechanism that can solve the strong CP problem, and introduces a new particle at the same time. Thanks to a popular brand of laundry detergent, this new curious particle was named "axion".
The emergence of axion also brought hopes to the hunt for dark matter. Astrophysicists, such as those at the International Axion Observatory (IAXO), have been gazing deep into the cosmos in search of boson particles that fit the description of axions. So far, they have not found anything that matches the Peccei-Quinn theory. But when they do, they could solve the two of the most prominent questions in physics at the same time.
Source: Seeker via Youtube