While background observables like the rate of expansion and the matter
content give us crucial information on the nature of dark energy, it is the growth of
matter perturbations that will be able to break degeneracies among the different
alternatives: quintessence, modified gravity, extra dimensions or even local curvature.
Future surveys like the Dark Energy Survey and Euclid will be able to chart the
Universe out to redshift z ~ 2, and allow us to disentangle the various alternatives.