Scalar-tensor theories of gravity provide a potential explanation for dark energy. However, any associated scalar fifth forces have not been observed to date in local tests of gravity. The coupling to matter must therefore be fine-tuned, or the scalar fifth force itself must somehow be screened in the local environment. In this talk, we focus on so-called symmetron models, wherein spontaneous symmetry breaking leads to density-dependent modifications of the matter coupling. In addition, we discuss the potential implications of the symmetron mechanism for astrophysical observations, illustrating that it is capable of describing observed galactic rotation curves without the need for particle dark matter.