  Positivity bounds - constraints on any low-energy effective field theory
imposed by the fundamental axioms of unitarity, causality and locality in the
UV - have recently been used to constrain scalar-tensor theories of dark
energy. However, the coupling to matter fields has so far played a limited
role. We show that demanding positivity when including interactions with
standard matter fields leads to further constraints on the dark energy
parameter space. We demonstrate how implementing these bounds as theoretical
priors affects cosmological parameter constraints and explicitly illustrate the
impact on a specific Effective Field Theory for dark energy. We also show in
this model that the existence of a standard UV completion requires that
gravitational waves must travel superluminally on cosmological backgrounds.
