2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/335Neutral hydrogen traces gravitational potentials. In the nearby universe, 21cm emission-line surveys show that the bulk of the HI resides in well-formed, optically-luminous galaxies. At high redshift, 21cm line absorption against background radio quasars occurs in gas-rich systems identified with the highest HI column densities -- the ``damped Lyman alpha'' quasar absorption-line systems. High spatial-resolution observations of the redshifted 21cm line absorbers measure sizes and kinematics of the neutral absorbers.8 pages, 2 figures. Invited review to appear in Proceedings of the XIXth Recontres de Moriond, on "Building the Galaxies: from the Primordial Universe to the Present," Editors F. Hammer, T.X. Thuan, V. Cayatte, B. Guiderdoni and J. Tran Thanh Van, Editions FrontieresAstrophysicstext