  We acquired ALMA ground state absorption profiles of HCO+ and other molecules
toward 33 extragalactic continuum sources seen toward the Galactic anticenter,
deriving N(H2) = N(HCO+)/3x10^{-9}. We observed J=1-0 CO emission with the IRAM
30m in directions where HCO+ was newly detected.
  HCO+ absorption was detected in 28 of 33 new directions and CO emission along
19 of those 28. The 5 sightlines lacking detectable HCO+ have 3 times lower
mean EBV and N(DNM). Binned in EBV, N(H2) and N(DNM) are strongly correlated
and vary by factors of 50-100 over the observed range EBV~0.05-1 mag, while
N(HI) varies by factors of only 2-3. On average N(DNM) and N(H2) are well
matched, and detecting HCO+ absorption adds little/no H2 in excess of the
previously inferred DNM. There are 5 cases where 2N(H2) < N(DNM)/2 indicates
saturation of the HI emission. For sightlines with \WCO > 1 K-\kms the CO-H2
conversion factor N(H2)/\WCO\ = 2-3x10^{20}\pcc/K-\kms is higher than derived
from studies of resolved clouds in gamma-rays.
  Our work sampled primarily atomic gas with a mean H2 fraction ~1/3, but the
DNM is almost entirely molecular. CO fulfills its role as an H2 tracer when its
emission is strong, but large-scale CO surveys are not sensitive to H2 columns
associated with typical values N(DNM) = 2-6x10^{20}\pcc. Lower \XCO\ values
from $\gamma$-ray studies arise in part from different definitions and usage.
Sightlines with \WCO\ \ge 1 K-\kms\ represent 2/3 of the H2 detected in HCO+
and detecting 90% of the H2 would require detecting CO at levels \WCO\~0.2-0.3
K-\kms
  For full abstract see the paper
