  A simple extension of the Standard Model consists of a scalar field that can
potentially constitute the dark matter (DM). Significant attention has been
devoted to probing light $\mathcal{O}(\lesssim 10~\rm{eV})$ scalar DM, with a
multitude of experimental proposals based on condensed matter systems as well
as novel materials. However, the previously overlooked effective in-medium
mixing of light scalars with longitudinal plasmons can strongly modify the
original sensitivity calculations of the direct detection experiments. We
implement the in-medium effects for scalar DM detection, using thermal field
theory techniques, and show that the reach of a large class of direct DM
detection experiments searching for light scalars is significantly reduced.
This development identifies setups based on Dirac materials and tunable plasma
haloscopes as particularly promising for scalar DM detection. Further, we also
show that scalars with significant boost with respect to halo DM, such as those
produced in the Sun, decay of other particles or by cosmic rays, will not
suffer from in-medium suppression. Hence, multi-tonne direct DM detection
experiments, such as those based on xenon or argon, also constitute a favorable
target. We also discuss scalar mediated DM scattering.
