The most popular solution to the problem is through the Peccei–Quinn mechanism. This introduces a new global anomalous symmetry which is then spontaneously broken at low energies, giving rise to a pseudo-Goldstone boson called an axion. The axion ground state dynamically forces the theory to be CP-symmetric by setting θ θ ¯ ¯ = 0 . Axions are also considered viable candidates for dark matter and axion-like particles are also predicted by string theory.