What observational technique, besides X-ray detection, provided crucial insights into mapping the missing baryonic material outside the disk?

Answer

Radio astronomy using ASKAP

Radio telescope arrays, such as ASKAP, were used to directly target and analyze specific spectral lines indicative of the missing baryonic material located just outside the main body of the galaxy.

What observational technique, besides X-ray detection, provided crucial insights into mapping the missing baryonic material outside the disk?
galaxyAstronomerdark mattermassMilky Way