It all started around 13.8 billion years ago with a big, cosmological “bang” that brought the universe suddenly and spectacularly into existence. Shortly after, the infant universe cooled dramatically and went completely dark. Then, within a couple hundred million years after the Big Bang, the universe woke up, as gravity gathered matter into the first stars and galaxies. Light from these first stars turned the surrounding gas into a hot, ionized plasma — a crucial transformation known as cosmic reionization that propelled the universe into the complex structure that we…
