When you look at the sky, it's like looking through a time machine. The further you look, the longer the light took to reach our eyes. The most powerful telescopes on Earth can see out to a distance of 13 billion light-years away; but any more distant, and the first stars hadn't ignited yet to illuminate the sky - a time called the "Dark Era". The afterglow of the Big Bang, the cosmic background radiation, is present across the entire night sky, and astronomers have figured out how to spot the shadows cast by intervening particles to begin understanding the distribution of matter in the Dark Era.