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! (Soon to be uploaded.)

My research work at Macquarie University and CSIRO dealt with spectroscopic analysis of data from CSIRO's Murriyang-Parkes radio telescope to measure the amount of molecular hydrogen present in a high-latitude Galactic molecular cloud using OH and CH tracers.