The universe is filled with galaxies that clump together like cosmic metropolises in the vast emptiness of space. Now astronomers have taken a bunch of baby pictures of these galaxy clusters, capturing them when they're just a couple billion years old (that's young, considering the universe is 13.8 billion years old). With more than 200 likely baby clusters, it's the biggest such haul ever, providing clues to dark matter and how galaxies form and evolve over time.

Galaxy clusters form the structural backbone of the universe—it's where all the stuff is. Astronomers have seen plenty of these clusters throughout the universe, but what's been elusive are the young clusters. The farther you look in space, the longer it takes the light to reach your eyes, and the farther back in time you're looking. So the youngest clusters have to be really distant, which makes them really dim. And because they're young, they haven't had much time to build a lot of bright stars, making them even harder to detect.

Clusters are also difficult to find because they occupy just a sliver of the enormous expanse of space. "These are what I would call the one-percent regions—these are the most concentrated regions in our entire universe," says David Koo, an astronomer at the University of California, Santa Cruz. "They're like the ultra wealthy parts of space."

Which is why astronomers have only found a few baby clusters. But thanks to the Planck satellite, which watches the entire sky, astronomers have located the 200 galaxy clusters at the edge of the cosmos, when the universe was just three billion years old. "In one fell swoop they suddenly have so many to study," says Koo, who wasn't involved in the new findings. "That's pretty impressive."

Planck's main task is to study the cosmic microwave background—the afterglow that bathed the universe after the big bang. But the satellite also measures light at other wavelengths, allowing it to pick up signals from early galaxies. So astronomers scoured the Planck data and found a couple hundred of these bright spots (marked by the black dots in the gorgeous image below). They then pointed the Herschel space telescope, which measures far infrared and wavelengths just under a millimeter, at each of these sources. When they zoomed in closer, the astronomers found that these regions of space were indeed extra dense, each containing at least about 10 young galaxies churning out new stars at a prodigious rate up to a 1,500 times greater than the Milky Way.

This image shows the entire universe the way Planck sees it. The white band in the center is the Milky Way, and the black dots mark the locations of the baby galaxy clusters. The insets surrounding it show a zoomed-in image of the clusters, with yellow contour lines indicating the density. ESA/Planck Collaboration

These still-forming galaxies are in their formative years. Like young humans, they're susceptible to the influence of their peers. Enormous black holes forming at the center of these galaxies may produce powerful jets that blow away gas in neighboring galaxies. Gas is the stuff stars are made of, so if the galaxy next to you is blasting your gas away, you won't be able to make stars. The gravitational tug of nearby galaxies can also strip you of stars and gas. But exactly how much does a galaxy's upbringing dictate its fate? Or is its future ruled mainly by innate properties like size and mass? The young clusters may hold answers. "You have these special rare regions where you can compare properties of these galaxies with properties of galaxies outside the region," Koo says.

Clusters are also the cosmic hubs of dark matter, that mysterious and invisible stuff that acts as gravitational glue binding all the galaxies together. "They may offer one of the best ways for us to learn what dark matter is," says Brenda Frye, an astronomer at the University of Arizona who was a part of the new research. Galaxies "are not just moving however they like—they don't have free will," she says. "They're responding to what the gravitational potential is telling it to do." In other words, by studying how galaxies in clusters move, astronomers can infer dark matter's gravitational influence, measure its properties, and learn the role it plays in a young galaxy's life.

Astronomers still have to confirm that all of the young clusters really are clusters, and not a mix of nearby and distant galaxies that just happened to be in the same field of view. They most likely are, but no one knows for sure until researchers do a comprehensive analysis. Then comes the work of studying each of the young clusters in detail. "This will take many years of follow-up observations to really understand fully," Koo says. "But certainly at this early stage it's an exciting prospect."