Scalar-tensor theories of gravity provide a consistent framework to accommodate an ultralight quintessence scalar field. While the equivalence principle is respected by construction, deviations from general relativity and standard cosmology may show up at nucleosynthesis, cosmic microwave background, and solar system tests of gravity. After imposing all the bounds coming from these observations, we consider the expansion rate of the Universe at weakly interacting massive particle decoupling, showing that it can lead to an enhancement of the dark matter relic density up to few orders of magnitude with respect to the standard case. This effect can have an impact on supersymmetric candidates for dark matter.
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