Have axions been observed?

Unlike the electroweak force (as shown in the famous k long and k short decay experiments), the strong force described by QCD (Quantum Chromodynamics) doesn't seems to violate CP parity, despite QCD allows CP-violation.

In 1977 a model was proposed by R. D. Peccei and H. R. Quinn to explain this. It essentially introduces a new broken symmetry. It's associated Goldstone Boson(essentially a new particle associated to a spontaneously broken symmetry) was called an axion, in an obvious reference to a brand of detergent.

One nice thing about this model is that the axions have no electric charge and interact very weakly with matter, making it an obvious candidate for the dark matter, specially because the theory suggest that axions were created abundantly in the Big Bang. While there haven't been any direct observations of axions, the experiments haven't ruled them out, either.

The PVLAS experiment found a tiny light polarization rotation in strong magnetic fields which seem to suggest the existence of axions. You can read an accessible account of the result here. A second experiment CAST might be able to detect axions from the Sun.

Despite the current results from PVLAS are suggestive at most, a direct detection of axions (maybe by CAST) will have a big impact on our understanding of dark matter.