  Efficient scattering into the exciton polariton ground state is a key
prerequisite for generating Bose-Einstein condensates and low-threshold
polariton lasing. However, this can be challenging to achieve at low densities
due to the polariton bottleneck effect that impedes phonon-driven scattering
into low-momentum polariton states. The rich exciton landscape of transition
metal dichalcogenides (TMDs) provides potential intervalley scattering pathways
via dark excitons to rapidly populate these polaritons. Here, we present a
microscopic study exploring the time- and momentum-resolved relaxation of
exciton polaritons supported by a \ce{MoSe2} monolayer integrated within a
Fabry-Perot cavity. By exploiting phonon-assisted transitions between
momentum-dark excitons and the lower polariton branch, we demonstrate that it
is possible to circumvent the bottleneck region and efficiently populate the
polariton ground state. Furthermore, this intervalley pathway is predicted to
give rise to, yet unobserved, angle-resolved phonon sidebands in
low-temperature photoluminescence spectra that are associated with
momentum-dark excitons. This represents a distinctive experimental signature
for efficient phonon-mediated polariton-dark-exciton interactions.
