More gas is sometimes inferred in molecular cloud complexes than is represented in HI or CO emission, and this is called dark neutral medium (DNM). Our aim is to extend a study of DNM along 13 directions in the outskirts of Chamaeleon by determining the atomic or molecular character of the DNM along a larger sample of sightlines. We acquired ALMA ground rotational state absorption profiles of HCO+ and other molecules toward 33 compact extragalactic continuum background sources seen toward the Galactic anticenter, deriving N(H_2_)=N(HCO+)/3 10^-9^ as before. We observed J=1-0 CO emission with the IRAM 30m telescope 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 five sightlines lacking detectable HCO+ have three times lower and . Binned in E_B-V_, N(H_2_) and N(DNM) are strongly correlated and vary by factors of 50=100 over the observed range E_B-V_~0.05-1mag, while N(HI) varies by factors of only 2-3. On average N(DNM) and N(H_2_) are well matched, and detecting HCO- absorption adds little to no H_2_ in excess of the previously inferred DNM. There are five cases where 2N(H_2_)=1K-km/s, the CO-H_2_ conversion factor N(H_2_)/W_CO_=2-3x10^20^cm^-2^/(1K-km/s) is higher than is derived from studies of resolved clouds in gamma-rays. Our work sampled primarily atomic gas with a mean H_2_ fraction ~1/3, but the DNM is almost entirely molecular. CO fulfills its role as an H_2_ tracer when its emission is strong, but large-scale CO surveys are not sensitive to H_2_ columns associated with typical values N(DNM)=2-6x10^20^cm^-2^. Lower X_CO_ values from gamma-ray studies arise in part from different definitions and usage. Sightlines with W_CO_>=1K-km/s represent 2/3 of the H_2_ detected in HCO+ and detecting 90% of the H_2_ would require detecting CO at levels W_co_~=0.2-0.3K-km/s.

Cone search capability for table J/A+A/675/A145/list (List of fits spectra)