  Modified gravity has garnered interest as a backstop against dark matter and
dark energy (DE). As one possible modification, the graviton can become
massive, which introduces a new scalar field - here with a Galileon-type
symmetry. The field can lead to a nontrivial equation of state (EOS) of DE
which is density-and-scale-dependent. Tension between Type Ia supernovae and
Planck could be reduced. In voids the scalar field dramatically alters the EOS
of DE, induces a soon-observable gravitational slip between the two metric
potentials, and develops a topological defect (domain wall) due to a nontrivial
vacuum structure for the field.
