  When modeling infrared or gamma-ray data as a linear combination of observed
gas tracers, excess emission has been detected compared to expectations from
known neutral and atomic gas as traced by HI and CO measurements, respectively.
This excess might correspond to an additional gas component. This so-called
"dark gas" (DG) has been observed in our Galaxy, as well as the Magellanic
Clouds. For the first time, we investigate the correlation between visible
extinction (Av) data and gas tracers on large scales in the solar neighborhood.
Our work focuses on both the solar neighborhood ($|b|>10\degr$), and the inner
and outer Galaxy, as well as on four individual regions: Taurus, Orion,
Cepheus-Polaris and Aquila-Ophiuchus. Thanks to the recent production of an
all-sky Av map, we first perform the correlation between Av and both HI and CO
emission over the most diffuse regions, to derive the optimal (Av/NH)^(ref)
ratio. We then iterate the analysis over the entire regions to estimate the
CO-to-H2 conversion factor as well as the DG mass fraction. The average
extinction to gas column-density ratio in the solar neighborhood is found to be
(Av/NH)^(ref)=6.53 10^(-22) mag. cm^2, with significant differences between the
inner and outer Galaxy. We derive an average XCO value of 1.67 10^(20) H2
cm^(-2)/(K km s^(-1)). In the solar neighborhood, the gas mass in the dark
component is found to be 19% relative to that in the atomic component and
164$%$ relative to the one traced by CO. These results are compatible with the
recent analysis using Planck data within the uncertainties of our measurements.
We estimate the ratio of dark gas to total molecular gas to be 0.62 in the
solar neighborhood. The HI-to-H2 and H2-to-CO transitions appear for Av
$\simeq$0.2 mag and Av$\simeq1.5$ mag, respectively, in agreement with
theoretical models of dark-H2 gas.
