"Approximately 300,000 years after the Big Bang, all matter consisted of three chemical elements: hydrogen, helium, and a bit of lithium. This matter spread out as the universe expanded and cooled down, and from the far-flung gas clusters the first stars ignited. Eventually, due to gravitational clustering, primordial galaxies formed. The earliest galaxies, which evolved in the first few hundred million years after the Big Bang, contained stars that...