A galaxy seems like a hard thing to miss – let alone 39 of them – but that's exactly what a team of astronomers has just discovered. So where have these countless stars been hiding? About 11.5 billion light-years away, in a part of the light spectrum that's invisible to many telescopes. The discovery of so many star-forming galaxies this old may call into question our understanding of galaxy formation in the early universe.

When we look up at the night sky, the starry landscape we can see is only telling part of the story. Stars that shine in visible light are only one small part of the entire spectrum – those that are extremely far away become invisible to us thanks to the expansion of the universe itself.

As the universe expands, distant stars and galaxies are moving away from us at tremendous speeds. The increasing distance between us and them physically stretches the light as it travels towards Earth, which makes the wavelengths longer and eventually turns it into infrared light.

While infrared light is invisible to the naked eye, some telescopes are designed to pick up those signals, revealing entire galaxies that have never been discovered before. And that's exactly what happened in this case.

Astronomers from the Institute of Astronomy at the University of Tokyo initially suspected there was something strange in infrared data gathered by the Spitzer Space Telescope. They looked closer using the Atacama Large Millimeter/submillimeter Array (ALMA) and the Very Large Telescope, both in Chile.

Sure enough, they discovered 39 massive galaxies that had never been seen before. Their light was extremely faint, thanks to being shrouded in dust and their incredible distance – some 11.5 billion light-years away. And since looking deep into space is like looking back in time, that distance means we're seeing these galaxies as they were billions of years ago, at a time when galaxies weren't thought to be that big.

"This is the first time that such a large population of massive galaxies was confirmed during the first 2 billion years of the 13.7-billion-year life of the universe," says Tao Wang, researcher on the study. "These were previously invisible to us. This finding contravenes current models for that period of cosmic evolution and will help to add some details, which have been missing until now."

While the light is pretty faint from our perspective, the team says that if you were standing on a planet in one of these galaxies, you'd see a very vibrant canvas of stars overhead.

"For one thing, the night sky would appear far more majestic," says Wang. "The greater density of stars means there would be many more stars close by appearing larger and brighter. But conversely, the large amount of dust means farther-away stars would be far less visible, so the background to these bright close stars might be a vast dark void."

Current telescopes will have a hard time studying these galaxies in more detail, since optical wavelengths are needed to analyze their spectra to determine what chemicals make them up. But the team says that upcoming instruments like the James Webb Space Telescope, due to launch in 2021, might be able to get a better look.

The research was published in the journal Nature.

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