  The decay and annihilation cross sections of dark matter particles may depend
on the value of a chameleonic scalar field that both evolves cosmologically and
takes different values depending on the local matter density. This possibility
introduces a separation between the physics relevant for freeze-out and that
responsible for dynamics and detection in the late universe. We investigate how
such dark sector interactions might be implemented in a particle physics
Lagrangian and consider how current and upcoming observations and experiments
bound such dark matter candidates. A specific simple model allows for an
increase in the annihilation cross section by a factor of $10^6$ between
freeze-out and today, while more complicated models should also allow for
scattering cross sections near the astrophysical bounds.
