These clouds are "visible" in the radio window through faint emission lines from some of the molecular species present in them, for instance OH and CH radicals.

OH and CH radicals show transitions at 18 and 9 cm wavelengths respectively. (Figures above showing some of the OH 18 cm hyperfine transitions (Left Figure) near 1.6 GHz and the CH hyperfine transitions (Right Figure) near 3.3 GHz.) 

OH and CH make a good couple! ❤️ Wondering why? I explore it in my thesis! (Link to text)

(Thesis Audio — Simplified for laymen, using AI. )

My PhD research at Macquarie University and CSIRO deals with understanding the role of simple hydrides in tracing the gas that other conventional tracers cannot trace.