  We investigate the temperature distribution of CO-dark molecular hydrogen
(H2) in a series of disk galaxies simulated using the AREPO moving-mesh code.
In conditions similar to those in the Milky Way, we find that H2 has a flat
temperature distribution ranging from 10 - 100 K. At $T < 30$ K the gas is
almost fully molecular and has a high CO content, whereas at $T > 30$ K, the H2
fraction spans a broader range and the CO content is small, allowing us to
classify gas in these two regimes as CO-bright and CO-dark, respectively. The
mean sound speed in the CO-dark H2 is 0.64 km/s, significantly lower than the
value in the cold atomic gas (1.15 km/s), implying that the CO-dark molecular
phase is more susceptible to turbulent compression and gravitational collapse
than its atomic counterpart. We further show that the temperature of the
CO-dark H2 is highly sensitive to the strength of the interstellar radiation
field, but that conditions in the CO-bright H2 remain largely unchanged.
Finally, we examine the usefulness of the [CII] and [OI] fine structure lines
as tracers of the CO-dark gas. We show that in Milky Way-like conditions,
diffuse [CII] emission from this gas should be detectable. However, it is a
problematic tracer of this gas, as there is only a weak correlation between the
brightness of the emission and the H2 surface density. The situation is even
worse for the [OI] line, which shows no correlation with the H2 surface
density.
