<human>:  Who discovered the interstellar medium?
<bot>: The interstellar medium was discovered by the astronomer Carl Sagan. He was the first to observe it, and he was also the first to propose that it might be a "cosmic ocean" of hydrogen and helium gas. He proposed this theory in the 1970s, but he didn't get much attention at the time because other astronomers thought that the gas was too cold to be in the form of a liquid.