Gas glows white, lit by a stellar nursery, in this view of a region within the Large Magellanic Cloud, the Milky Way’s largest satellite galaxy. Most cosmic gas is not so visible and lies outside of galaxies — in halos surrounding galaxies and in the vast spaces in between. Yet the gas determines galactic life cycles. (Credit: ESA/Hubble & NASA)

Most of what astronomers know about the universe comes from what they can see. So their ideas have been prejudiced toward stars and galaxies, w