Most of what astronomers know about the universe comes from what they can see. So their ideas have been prejudiced toward stars and galaxies, which are bright. But most of the regular matter in the universe is in the form of gas, which is dim. Gas called the intergalactic medium fills the space between galaxies; the gas of the circumgalactic medium surrounds galaxies more closely. The gas in both places regulates the birth, life and death of the galaxies, and holds a detailed history of the universe. Only lately have astronomers been able to detect it.
Shortly after its birth, the universe was filled with gas, mostly hydrogen. Over time, here and there, gravity pulled the gas into clouds which turned into galaxies and in which stars ignited. Stars shine by thermonuclear burning of the gas; of those that die in explosions, some blow the gas back out of the galaxies. Out in intergalactic space, the gas cools and gets denser, until gravity pulls it back into the galaxy where new stars form. The process repeats: Gravity condenses gas into galaxies and stars, stars blow up and kick the gas out, gravity cycles the gas back in and makes new stars.
In time, any given galaxy begins to run out of recyclable gas. Without gas, it can’t form new stars; the old stars live out their lives and die, and eventually the galaxy dies too. Galaxies sit in a bath of gas, the medium from which they were born and which fuels them. The galaxies breathe gas in and out, and their stars burn until their gas is gone.
This is theory. The problem with verifying it has been that astronomers’ instruments could barely detect signs of gas, let alone map its comings and goings. With more sensitive instruments and dogged surveys, astronomers now know more. Convincing evidence suggests that the intergalactic medium is rich in gas, which fills the universe and seeds galaxies. Less-convincing and sometimes puzzling evidence in the circumgalactic medium shows that galaxies live by recycling gas into and out of stars. And astronomers have only preliminary evidence supporting arguments for how galaxies might run out of gas, stop forming stars and die.
