  We present experimental and theoretical results on the high-quality
single-layer MoS$_{2}$ which reveal the fine structure of charged excitons,
i.e., trions. In the emission spectra we resolve and identify two trion peaks,
T$_{1}$ and T$_{2}$, resembling the pair of singlet and triplet trion peaks
(T$_S$ and T$_{T}$) in tungsten-based materials. However, in
polarization-dependent photoluminescence measurements we identify these peaks
as novel intra- and inter-valley singlet trions, constituting the trion fine
structure distinct from that already known in bright and dark 2D materials with
large conduction-band splitting induced by the spin-orbit coupling. We show
that the trion energy splitting in MoS$_{2}$ is a sensitive probe of inter- and
intra-valley carrier interaction. With additional support from theory we claim
that the existence of these singlet trions combined with an anomalous excitonic
g-factor and the characteristic temperature dependence of the emission spectra
together suggest that monolayer MoS$_{2}$ has a dark excitonic ground state,
despite having "bright" single-particle arrangement of spin-polarized
conduction bands.
