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21-cm experiments are surveys of redshifted 21-cm neutral hydrogen emission lines from the epoch of reionization. A goal is to observe the effects of intervening matter (plasma) in galaxy clusters, etc., to draw conclusions about the placement of such objects that are further than observation has allowed, i.e., to map much more of the universe and its history. The 21-cm line is due to the difference between two neutral hydrogen ground states. Reionization occurred between z of 6 and 20, so a redshifted 21-cm signal from that era will be in the 1-4 meter range. Some efforts:
The latter two make no attempt to map it but merely to detect it. As of 2023, the only detection claim I'm aware of is regarding EDGES, though the claim has been challenged. Other than that, the only result so far has been additional information regarding the challenge of detecting the signal, i.e., how faint it apparently is.
I ran across the term 21-cm experiment specifically meaning the above efforts, i.e., detection of it produced during the EoR. Another type of survey performs intensity mapping of the 21-cm line from lesser redshifts, with the goals of mapping the universe through detection of the HI regions within otherwise-undetected galaxies, and detection of the consequences of baryon acoustic oscillations (BAOs):
(Some of these have adopted the additional goal of FRB-study because the equipment also happens to detect FRBs.) There have also been efforts to map the 21-cm line within the Milky Way and nearer galaxies: