Chandra Shekhar Murmu

The universe was born dark and cold, like a room with the lights off. Then someone started turning on the bulbs — the first stars. Around each such star, gas heated up and lost electrons, becoming transparent to light. These glowing regions grew like bubbles in boiling water and gradually merged until the entire cosmos became transparent.

Long before stars appeared, the universe was filled with cold hydrogen. This gas can 'sing'—emitting radio waves with a wavelength of 21 centimeters. When the first stars and galaxies ignited, their ultraviolet light began to strip electrons from hydrogen atoms, turning neutral gas into ionized gas.