  In this work we study the electronic structure of Ag${}_3$AuSe${}_2$ and
Ag${}_3$AuTe${}_2$, two chiral insulators whose gap can be tuned through small
changes in the lattice parameter by applying hydrostatic pressure or choosing
different growth protocols. Based on first principles calculations we compute
their band structure for different values of the lattice parameters and show
that while Ag${}_3$AuSe${}_2$ retains its direct narrow gap at the $\Gamma$
point, Ag${}_3$AuTe${}_2$ can turn into a metal. Focusing on Ag${}_3$AuSe${}_2$
we derive a low energy model around $\Gamma$ using group theory, which we use
to calculate the optical conductivity for different values of the lattice
constant. We discuss our results in the context of detection of light dark
matter particles, which have masses of the order of a $k$eV, and conclude that
Ag${}_3$AuSe${}_2$ satisfies three important requirements for a suitable
detector: small Fermi velocities, $m$eV band gap and low photon screening. Our
work motivates the growth of high-quality and large samples of
Ag${}_3$AuSe${}_2$ to be used as target materials in dark matter detectors.
