21cm 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 further than observation has allowed, i.e., map much more of the universe and its history.
The 21cm line comes from the difference between two neutral hydrogen ground states. Reionization happened between z of 6 and 20, so a redshifted 21 cm signal from that time will be in the 1-4M range. The Murchison Widefield Array (MWA), Low-frequency Array (LOFAR), PAPER, and HERA are looking at this. As of 2016, this signal has yet to be isolated, but efforts continue to find it.