  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.
