  The CO-dark molecular gas (DMG), which refers to the molecular gas not traced
by CO emission, is crucial for the evolution of the interstellar medium (ISM).
While the gas properties of DMG have been widely explored in the Solar
neighborhood, whether or not they are similar in the outer disk regions of the
Milky Way is still not well understood. In this Letter, we confirm the
existence of DMG toward a cold HI arc structure at 13 kpc away from the
Galactic center with both OH emission and HI narrow self-absorption (HINSA).
This is the first detection of HINSA in the outer disk region, in which the
HINSA fraction ($N_{\rm HINSA}$/$N_{\rm H_2}$ = 0.022$\pm$0.011) is an order of
magnitude higher than the average value observed in nearby evolved dark clouds,
but is consistent with that of the early evolutionary stage of dark clouds. The
inferred H$_2$ column density from both extinction and OH emission ($N_{\rm
H_2} \approx 10^{20}$ cm$^{-2}$) is an order of magnitude higher than
previously estimated. Although the ISM environmental parameters are expected to
be different between the outer Galactic disk regions and the Solar
neighborhood, we find that the visual extinction ($A_{\rm V}$ = 0.19$\pm$0.03
mag), H$_2$-gas density ($n_{\rm H_2} = 91\pm46$ cm$^{-3}$), and molecular
fraction (58\%$\pm$28\%) of the DMG are rather similar to those of nearby
diffuse molecular clouds. The existence of DMG associated with the expanding HI
supershell supports a scenario where the expansion of supershells may trigger
the formation of molecular clouds within a crossing timescale of the shock wave
($\sim$10$^6$ yr).
