  The conversion factor from carbon monoxide (CO) intensity to molecular gas
mass is a source of large uncertainty in understanding gas and its relation to
star formation in galaxies. In particular, the conversion factor in low
metallicity environments have remained elusive, as currently only two galaxies
have been detected in any CO isotopes in environments with 12+log (O/H) < 8.0.
Here we report 12CO(J=1-0) and 13CO(J=1-0) observations towards a star forming
region in DDO 154, a low metallicity dwarf irregular galaxy at 12+log (O/H) =
7.67. This is a re-observation of a previous non-detection at higher angular
and velocity resolution. No significant emission was detected. By estimating
the molecular gas mass from associated star formation, we find that DDO 154 has
a conversion factor of more than 10^3 times the Milky Way. Alternatively, if we
estimate molecular mass using dust continuum emission, the conversion factor is
at least 2 orders of magnitude larger than the Milky Way. These estimates
signify a large amount of CO-dark molecular gas in this galaxy.
