  Charge carriers in two-dimensional transition metal dichalcogenides (TMDs),
such as WSe$_2$, have their spin and valley-pseudospin locked into an
optically-addressable index that is proposed as a basis for future information
processing. The manipulation of this spin-valley index requires tuning its
energy, typically through external magnetic field (B), which is cumbersome.
Thus, other efficient routes like all-optical control of spin-valley index are
desirable. Here, we show that many-body interactions amongst interlayer
excitons in WSe$_2$/MoSe$_2$ heterobilayer induce a steady-state valley Zeeman
splitting corresponding to B $\sim$ 6 Tesla. This anomalous splitting, present
at incident powers as low as $\mu$Ws, increases with power and enhances,
suppresses or even flips the sign of a B-induced splitting. Moreover, the
$\it{g}$-factor of valley Zeeman splitting can be tuned by $\sim$ 30 $\%$ with
incident power. In addition to valleytronics, our results are relevant for
achieving optical non-reciprocity using two-dimensional materials.
