  Employing the original, all-optical method, we quantify the magnetic
susceptibility of a two-dimensional electron gas (2DEG) confined in the
MoSe$_2$ monolayer in the range of low and moderate carrier densities. The
impact of electron-electron interactions on the 2DEG magnetic susceptibility is
found to be particularly strong in the limit of, studied in detail, low carrier
densities. Following the existing models, we derive the value of $g_0 = 2.5 \pm
0.4$ for the bare (in the absence of the interaction effects) $g$-factor of the
ground state electronic band in the MoSe$_2$ monolayer. The derived value of
this parameter is discussed in the context of estimations from other
experimental approaches. Surprisingly, the conclusions drawn differ from
theoretical ab-initio studies.
