What observational technique allows astronomers to map the distribution of neutral hydrogen gas extending beyond dust obscuration?

Answer

Radio astronomy observing the 21-cm spectral line

Radio telescopes sensitive to the 21-cm spectral line of neutral hydrogen can penetrate the opaque dust clouds that block visible light, mapping the raw fuel supply across the disk.

What observational technique allows astronomers to map the distribution of neutral hydrogen gas extending beyond dust obscuration?
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