The universe has plenty of hydrogen. So why is star formation collapsing?

Star formation across the universe has fallen dramatically over the past 4.5 billion years, but astronomers have uncovered a surprising twist: the supply of neutral hydrogen, a key ingredient for making stars, has barely declined. Using China’s FAST radio telescope and data from roughly 2.5 million galaxies observed by DESI, researchers found that star formation was about 2.5 times higher 4.5 billion years ago, while neutral hydrogen levels were only about 1.4 times higher.