  We report the observation and gate manipulation of intrinsic dark trions in
monolayer WSe$_2$. By using ultraclean WSe$_2$ devices encapsulated by boron
nitride, we directly resolve the weak photoluminescence of dark trions. The
dark trions can be tuned continuously between negative and positive charged
trions with electrostatic gating. We also reveal their spin triplet
configuration and distinct valley optical emission by their characteristic
Zeeman splitting under magnetic field. The dark trions exhibit large binding
energy (14-16 meV). Their lifetime (~1.3 ns) is two orders of magnitude longer
than the bright trion lifetime (~10 ps) and can be tuned between 0.4 to 1.3 ns
by electrostatic gating. Such robust, optically detectable, and gate tunable
dark trions provide a new path to realize electrically controllable trion
transport in two-dimensional materials.
