  The remarkably strong Coulomb interaction in atomically thin transition metal
dichalcogenides (TMDs) results in an extraordinarily rich many-particle physics
including the formation of tightly bound excitons. Besides optically accessible
bright excitonic states, these materials also exhibit a variety of dark
excitons. Since they can even lie below the bright states, they have a strong
influence on the exciton dynamics, lifetimes, and photoluminescence. While very
recently, the presence of dark excitonic states has been experimentally
demonstrated, the origin of these states, their formation, and dynamics have
not been revealed yet. Here, we present a microscopic study shedding light on
time- and energy-resolved formation and thermalization of bright and dark
intra- and intervalley excitons as well as their impact on the
photoluminescence in different TMD materials. We demonstrate that intervalley
dark excitons, so far widely overlooked in current literature, play a crucial
role in tungsten-based TMDs giving rise to an enhanced photoluminescence and
reduced exciton lifetimes at elevated temperatures.
