The end of unified dark matter?
  Despite the interest in dark matter and dark energy, it has never been shown that they are in fact two separate substances. We provide the first strong evidence that they are separate by ruling out a broad class of so-called unified dark matter models that have attracted much recent interest. We find that they produce oscillations or exponential blowup of the matter power spectrum inconsistent with observation. For the particular case of generalized Chaplygin gas models, 99.999% of the previously allowed parameter space is excluded, leaving essentially only the standard Lambda-CDM limit allowed. 
Havard Sandvik (Penn), Max Tegmark (Penn), Matias Zaldarriaga (NYU),   Ioav Waga (UFRJ)