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 be- lieved to be the main gas component of the intermediate extinction region from Av~0.05-2, roughly corresponding to the self-shielding threshold of H2 and 13CO. To quantify DMG relative to HI and CO, we are pursuing three observational techniques; HI self-absorption, OH absorption, and THz C+ emission. In this paper, we focus on preliminary results from a CO and OH absorption survey of DMG candidates. Our analysis shows that the OH excitation temperature is close to that of the Galactic continuum back- ground and that OH is a good DMG tracer co-existing with molecular hydrogen in regions without CO. Through systematic "absorption mapping" by the 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.

ABSTRACT
1. INTRODUCTION
2. WHERE IS THE HYDROXYL?
3. A MULTI-TRANSITION CO SURVEY OF MILLEN-NIUM SOURCES
4. COMPARISON OF HI, OH, AND CO
5. DISCUSSION
REFERENCES

• Di Li(National Astronomical Observatories, Chinese Academy of Sciences/Key Laboratory for Radio Astronomy, Chinese Academy of Sciences)
• Duo Xu(National Astronomical Observatories, Chinese Academy of Sciences)
• Carl Heiles(Astronomy Department, University of California, Berkeley)
• Zhichen Pan(National Astronomical Observatories, Chinese Academy of Sciences/Key Laboratory for Radio Astronomy, Chinese Academy of Sciences)
• Ningyu Tang(National Astronomical Observatories, Chinese Academy of Sciences)