After the universe emerged from the Big Bang 13.8 billion years ago, galaxies filled with stars began to form relatively quickly around 3 billion years later. There was plenty of gas to go around, so a small portion of these early galaxies were able to grow into massive, hyper-luminous galaxies, with a brightness of 10 trillion suns. As the gas reserves depleted with time, fewer galaxies could grow at a fast pace.
When astronomers observed the universe with the infrared space telescope Herschel, they found that this theory largely checks out. However, in terms of absolute numbers, it looked like there are over an order of magnitude too many hyper-luminous infrared galaxies, both in the early universe and more recent epochs. Unfortunately, Herschel’s spatial resolution couldn’t resolve all individual galaxies, so they couldn’t say for sure.