Astronomers just mapped one of the largest structures in the universe, long hidden behind the Milky Way's 'Zone of Avoidance'

Astronomers have finally mapped out a mysterious galactic "supercluster" that has been almost completely hidden from Earth since it was discovered 10 years ago. The results reveal that the structure is far larger than we realized, and it now ranks among some of the most massive objects in the known universe.

The Vela Supercluster is a collection of at least 20 galaxy clusters, each of which contains hundreds or thousands of galaxies, all gravitationally bound into a single entity. Despite its immense size, the supercluster was just discovered in 2016 because of its location: It lies around 800 million light-years from Earth within a region that experts call the "Zone of Avoidance" — the part of the night sky where we see the Milky Way, which is so full of stars and dust that it's almost impossible to view anything behind it.

The supercluster was initially dubbed "Terra incognita," which means "unknown land" in Latin, and its cosmic hiding place had prevented researchers from determining its true size.

But in a new study, uploaded March 10 to the preprint server arXiv, researchers loosely mapped the Vela Supercluster by measuring the movements of galaxies within and around the galactic swarm's edge.

Their findings revealed that the structure is around 300 million light-years across — about 3,000 times wider than the Milky Way — and contains an enormous amount of matter equivalent to about 30 quadrillion suns, the researchers wrote in a statement. The new map also shows that most of this mass is distributed into two cores that are moving toward each other.

The team was "delighted" to confirm that the supercluster is "a coherent large-scale structure comparable in size and mass to some of the largest and well-known superclusters in the local universe," study co-author Renee Kraan-Korteweg, an astronomer at the University of Cape Town who specializes in the Zone of Avoidance, told Live Science in an email.

The Vela Supercluster now ranks as more massive than Laniākea, the supercluster that contains Earth and the rest of our galaxy, and is "a close second" to the Shapley Supercluster, which is widely considered the largest supercluster of galaxies, Kraan-Korteweg said. (Other structures, such as the Hercules-Corona Borealis Great Wall and the recently discovered "Quipu," are even larger, but they are considered a step above a supercluster — essentially, clusters of superclusters.)

The researchers also gave the supercluster a new nickname: Vela-Banzi, which means "revealing widely" in the Xhosa language used by Indigenous people in South Africa, where most of the telescopes used in the study are based.

Peering through the Milky Way

The Zone of Avoidance has long frustrated astronomers who want to know what lies behind the thick disk of stars, gas and dust of the Milky Way, which covers up to 20% of the visible night sky.

"The millions/billions of stars forming the disk are so dense [and so] close to the galactic plane that we cannot easily see through it," Kraan-Korteweg wrote. "Moreover, where we have stars, we also have lots of minuscule dust particles, and like the stars, this dust layer gets thicker and thicker as you approach the plane."

To get around this, the researchers combined 65,000 existing galaxy distance measurements with around 8,000 new redshift observations of other galaxies. (Redshift measures how fast something is moving away from Earth by calculating how much its light has been stretched by the expansion of the universe.)

Of these observations, the most important were around 2,000 redshift measurements captured by South Africa's MeerKAT telescope, which detects radio waves shining from the giant clouds of hydrogen gas that permeate most galaxies. This allowed the team to take direct measurements of galaxy movements within Vela that had never been seen in visible light.

The researchers think it may be possible to create more accurate maps of the Vela Supercluster with more powerful radio telescopes in the future. However, not all galaxies contain large amounts of hydrogen that we can see, so parts of the structure will likely always "remain partly shrouded to us," Kraan-Korteweg said.

A better understanding of the largest structures in the universe will help astronomers confirm their models of cosmology. However, to do this, they need to know both the size and the speed of these objects, which is why the new findings are so exciting.

"To understand the one, we need to know the other," Kraan-Korteweg said. "And if we have both, we will be [able] to check if we can reconcile these observations with the models of the universe."