In astronomy what is 21-cm radiation With what element is it associated?

21-cm radiation refers to the electromagnetic radiation emitted by neutral hydrogen atoms (H I) when their electron transitions between two hyperfine energy levels. This specific wavelength, approximately 21 centimeters, is significant in radio astronomy as it allows scientists to observe and map the distribution of hydrogen in the universe, helping to study the structure and dynamics of galaxies and interstellar medium. It serves as a crucial tool for understanding cosmic phenomena and the large-scale structure of the cosmos.