A new paper has thrown a wrinkle into the dark energy puzzle. A "chameleon" particle has been proposed (link open only for a limited time) that can arise out of photon traveling from distant astronomical bodies while passing through magnetic fields, very much like the proposed axions, which have yet to be detected. However, the authors of this proposed chameleon particles claim that there is a "good evidence" that they may have been detected already.
In theory, photons that travel through magnetic fields can turn into chameleons, reducing the amount of light that reaches Earth from distant sources. The amount of dimming depends on the light's frequency. By comparing light emitted across a range of frequencies from the luminous centres of 77 active galaxies, Douglas Shaw at Queen Mary University of London and his colleagues have found what they call "good evidence" that some photons have gone missing in transit.
However, these chameleon particles cannot be distinguished from those axions.
By themselves, the observations of dimmed light by Shaw and his colleagues can't distinguish between models that rely on chameleons and models in which photons turn into other 'axion-like' particles. Either "would be an interesting discovery," says Shaw.
However, only the chameleon model predicts that the photons' polarizations should be aligned with the magnetic fields they traversed. So far, the team has studied data on light from three stars in the Milky Way galaxy and in each case found the required polarization3.
Looks like there are people who are already planning on testing this in the next couple of years, so it should be interesting to see how this turns out.
 C. Burrage et al., Phys. Rev. Lett. v.102, p.201101 (2009).