Quantifying Dark Gas
  A growing body of evidence has been supporting the existence of so-called
"dark molecular gas" (DMG), which is invisible in the most common tracer of
molecular gas, i.e., CO rotational emission. DMG is believed to be the main gas
component of the intermediate extinction region between A$\rm_v$$\sim$0.05-2,
roughly corresponding to the self-shielding threshold of H$_2$ and $^{13}$CO.
To quantify DMG relative to HI and CO, we are pursuing three observational
techniques, namely, HI self-absorption, OH absorption, and TeraHz C$^+$
emission. In this paper, we focus on preliminary results from a CO and OH
absorption survey of DMG candidates. Our analysis show that the OH excitation
temperature is close to that of the Galactic continuum background and that OH
is a good DMG tracer co-existing with molecular hydrogen in regions without CO.
Through systematic "absorption mapping" by Square Kilometer Array (SKA) and
ALMA, we will have unprecedented, comprehensive knowledge of the ISM components
including DMG in terms of their temperature and density, which will impact our
understanding of galaxy evolution and star formation profoundly.
