Millimeter-wave (mm-wave) absorption profiles toward extragalactic sources consistently find just diffuse (occasionally perhaps translucent) neutral gas—low/moderate density and extinction—along even some very long, dark lines of sight. CO, often heavily fractionated and mimicking the appearance of dark gas in emission, occasionally absent in emission even when present in absorption, is not the dominant form of carbon in these regions (presumably it is C+) yet the abundances of many other molecules resemble those seen in TMC-1. Some species (OH, HCO+, C2H and C3H2) turn on with high abundances just when H2 does; others (HCN, HNC, CN) require slightly higher $N$(H2) and yet others (CS and other sulfur-bearing species,NH3 and H2CO) even higher $N$(H2). The systematics and implications of these recent discoveries are discussed here.
