  We perform absorption and photoluminescence spectroscopy of trions in
hBN-encapsulated WSe$_2$, WS$_2$, MoSe$_2$, and MoS$_2$ monolayers, depending
on temperature. The different trends for W- and Mo-based materials are
excellently reproduced considering a Fermi-Dirac distribution of bright and
dark trions. We find a dark trion, $\rm{X_D^-}$ 19 meV $\textit{below}$ the
lowest bright trion, $\rm{X}_1^-$ in WSe$_2$ and WS$_2$. In MoSe$_2$,
$\rm{X_D^-}$ lies 6 meV $\textit{above}$ $\rm{X}_1^-$, while $\rm{X_D^-}$ and
$\rm{X}_1^-$ almost coincide in MoS$_2$. Our results agree with GW-BSE
$\textit{ab-initio}$ calculations and quantitatively explain the optical
response of doped monolayers with temperature.
