  We consider self-annihilation of dark matter, $\chi$, into metastable
mediators, $Y$, and their subsequent decay into photons inside white dwarfs. We
focus on reactions of the type $\chi \bar{\chi}\rightarrow YY$, where
mediators, besides having a finite decay lifetime at rest $\tau_{\rm
rest}\lesssim 1$ s, may suffer energy loss in the medium before they decay into
photons, $Y \rightarrow \gamma\gamma$. We obtain attenuated gamma-ray
luminosities arising from the combination of both effects. Using complementary
sets of astrophysical measurements from cold white dwarfs in the M4 globular
cluster as well as direct/indirect dark matter searches we discuss further
constraints on dark mediator lifetimes.
