Abstract: Attempts to explain the current acceleration of the universe discovered with type Ia supernovae have generated bizarre ideas: dark energy, backreaction of inhomogeneities, or giant cosmic voids. A popular alternative consists of modifying gravity at cosmological scales. But this is risky: even when everything goes well for cosmology, other fundamental and experimental aspects of gravity must be looked at in order for the theory to be viable. The successes of, and challenges for, modified gravity will be reviewed.
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