= Solution
Cold <molecular hydrogen> is difficult to observe directly: its homonuclear symmetry forbids electric-dipole rotational transitions, and its lowest useful quadrupole rotational excitation is high compared with a cold cloud's <temperature>. <Carbon monoxide> is relatively abundant, has a permanent dipole moment and has low-lying rotational transitions that can be collisionally excited in cold gas. The low-$J$ lines lie in accessible millimetre bands and suffer much less dust extinction than visible light. They therefore reveal molecular-gas velocities and spatial structure efficiently.
The advantages do not turn CO into a direct <mass> counter without a model. Common-isotope lines can be optically thick; excitation can be subthermal or have multiple <temperatures>. Abundance changes with metallicity, chemistry, shielding and photodissociation, and CO can freeze onto grains. <Molecular hydrogen> can also survive where CO is weak, giving CO-dark gas. A calibrated <carbon-monoxide tracer of molecular hydrogen> conversion may be useful for appropriate cloud populations but should not be transplanted uncritically to newly formed supernova ejecta. The line calculations here infer CO <mass>, not total molecular <mass> unless a CO-to-hydrogen abundance is separately known.
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