  The $\rm H_2$ mass of molecular clouds has traditionally been traced by the
CO(J=1-0) rotational transition line. This said, CO is relatively easily
photodissociated, and can also be destroyed by cosmic rays, thus rendering some
fraction of molecular gas to be "CO-dark". We investigate the amount and
physical properties of CO-dark gas in two $z \sim 0$ disc galaxies, and develop
predictions for the expected intensities of promising alternative tracers ([CI
609 $\mu$m and [CII] 158 $\mu$m emission). We do this by combining cosmological
zoom simulations of disc galaxies with thermal-radiative-chemical equilibrium
interstellar medium (ISM) calculations to model the predicted H~\textsc{i} and
$\rm H_2$ abundances and CO(J=1-0), [CI] 609 $\mu$m and [CII] 158 $\mu$m
emission properties. Our model treats the ISM as a collection of radially
stratified clouds whose properties are dictated by their volume and column
densities, the gas-phase metallicity, and the interstellar radiation field and
cosmic ray ionization rates. Our main results follow. Adopting an
observationally motivated definition of CO-dark gas, i.e. $\rm H_2$ gas with
$W_{\rm CO} < 0.1 $ K-km/s, we find that a significant amount ($> 50\%$) of the
total $\rm H_2$ mass lies in CO-dark gas, most of which is diffuse gas, poorly
shielded due to low dust column density. The CO-dark molecular gas tends to be
dominated by [CII], though [CI] also serves as a bright tracer of the dark gas
in many instances. At the same time, [CII] also tends to trace neutral atomic
gas. As a result, when we quantify the conversion factors for the three
carbon-based tracers of molecular gas, we find that [CI] suffers the least
contamination from diffuse atomic gas, and is relatively insensitive to
secondary parameters.
