  The most abundant interstellar molecule, molecular Hydrogen (H$_{2}$), is
practically invisible in cold molecular clouds. Astronomers typically use
carbon monoxide (CO) to trace the bulk distribution and mass of H$_{2}$ in our
galaxy and many others. CO observations alone fail to trace a massive component
of molecular gas known as "CO-dark" gas. We present an ultra sensitive pilot
search for the 18cm hydroxyl (OH) lines in the Andromeda Galaxy (M31) with the
100m Robert C. Byrd Green Bank Telescope. We successfully detected the 1667 and
1665 MHz OH in faint emission. The 1665/1667 MHz line ratio is consistent with
the characteristic 5:9 ratio associated with local thermodynamic equilibrium
(LTE). To our knowledge, this is the first detection of non-maser 18cm OH
emission in another galaxy. We compare our OH and HI observations with archival
CO (1-0) observations. Our OH detection position overlaps with the previously
discovered Arp Outer Arm in CO. Our best estimates show that the amount of
H$_{2}$ traced by OH is 140% higher than the amount traced by CO in this
sightline. We show that the amount of dark molecular gas implied by dust data
supports this conclusion. We conclude that the 18cm OH lines hold promise as a
valuable tool for mapping of the "CO-dark" and "CO-faint" molecular gas phase
in nearby galaxies, especially with upcoming multi-beam, phased-array feed
receivers on radio telescopes which will allow for drastically improved mapping
speeds of faint signals.
