Last year, physicists reported seeing tantalizing experimental traces of the axion, a hypothetical subatomic particle that's been mentioned as a possible constituent of cosmic dark matter. But the axion was showing up where theory said it shouldn't be. It now looks as if it wasn't there after all.
From Science News.
Explaining the effects described by lambda cold dark matter theory in cosmology requires one of two extraordinary conclusions: either General Relativity is wrong in weak fields, or there is a non-baryonic particle (i.e. one without protons or neutrons) out there that doesn't behave like any known subatomic particle.
Several variants of the former are lumped under the notion of "MOND" and were dealt a serious blow from the "Bullet Cluster" data. The Axion was a leading candidate for the latter. But, experimental results contradicting the first possible glimpse of the axion strongly suggest that the latter has problems.
The empircal evidence of dark matter pheneomena is overwhelming, but the lack of a mechanism for it that can be explained without new physics is troubling (or exciting, depending upon your personality).