About 1 billion years after the Big Bang, something weird happened in the universe. It mysteriously went from being dark to transparent. Now astronomers might know why.

A study published in Monthly Notices of the Royal Astronomical Society, and led by Philip Kaaret from the University of Iowa, found that black holes might have been responsible for the strange occurrence. The researchers believe that black holes fling out matter violently enough to pierce their surroundings. This would have allowed light to escape into the early universe.

When our universe first came into existence a whopping 13.8 billion years ago, it entered a period known as the cosmic dark ages. Filled with neutral hydrogen, most of the universe would have been unobservable until 500 million years after the Big Bang, when stars and galaxies started to appear. After another 500 million years, the universe became clear – entering a period known as the Epoch of Reionization (EOR).

The cause of the EOR has been the subject of much debate. But now we might have a definitive answer.

The researchers studied a galaxy called Tol 1247-232, which is located 600 million light-years from Earth. They found that an X-ray source in the galaxy was increasing and decreasing in brightness, and deduced that this must have been a black hole. They also found that ultraviolet light could pierce the cloudy galaxy, suggesting how the EOR happened. This may have been due to the black hole. 

“If true, it means that black holes helped us see back into the early universe, an unexpected role for objects that suck light in,” Kaaret told IFLScience.

Exactly how black holes eject matter isn’t properly understood. After all, beyond the boundary of a black hole, nothing – not even light – can escape its gravitational pull.

We know that black holes can produce powerful jets – we can even see these coming from the supermassive black hole at the center of our own galaxy. Traveling at the speed of light, it’s thought these jets are powered by rotation of the black hole itself. The material comes from a swirling disk of hot gas and dust, called the accretion disk, which surrounds some black holes.

“As matter falls into a black hole, it starts to spin and the rapid rotation pushes some fraction of the matter out,” Kaaret said in a statement. “They’re producing these strong winds that could be opening an escape route for ultraviolet light. That could be what happened with the early galaxies.”