Astronomers studying the largest detected structure in the universe, 1.8 billion light years wide, say they've figured out what it is — or actually isn't, in a sense, because it's a giant and unusually empty hole.

It was discovered in an astronomical survey trying to discover why around 10,000 expected galaxies were missing from the portion of the sky being observed, they say.

Dubbed a "supervoid" and distinctive for its unexpected emptiness, it is "the largest individual structure ever identified by humanity," says study leader István Szapudi of the University of Hawaii at Manoa.

His research team was looking at the region that was previously found to be an area where the cosmic microwave background radiation is much cooler that the average surrounding background temperatures, which astronomers quickly named the Cold Spot.

The existence of such a large, cold area was out of step with most models of the evolution of the universe after the Big Bang, which allowed for some warmer and cooler regions to emerge in the early days of the universe, but nothing on the scale of the Cold Spot.

The giant region of emptiness detected may at least partly explain the very existence of the Cold Spot, the researchers suggest, with the supervoid sucking energy out of the cosmic background radiation that travels across it.

The supervoid associated with the Cold Spot is not completely empty — it's not a vacuum, the researchers explain — but it possesses around 20 percent less material in it than any typical region of the universe.

"Supervoids are not entirely empty, they're under-dense," says study co-author András Kovács of the Eötvös Loránd University in Budapest.

This supervoid, located 3 billion light years from Earth — comparatively close in the cosmic scale of distances — was detected by analyzing data from the Pan-STARRS1 telescope located on Maui in Hawaii and from NASA's Wide Field Survey Explorer (WISE) satellite.

The supervoid is unexpected given the usual even distribution of the universe at the scale the empty region occupies, the researchers say.

"This is the greatest supervoid ever discovered," says Kovács. "In combination of size and emptiness, our supervoid is still a very rare event. We can only expect a few supervoids this big in the observable universe."

While strongly suggesting an association between the supervoid and the Cold Spot, the new study hasn't completely proved a link, the researchers acknowledge, although they point out that two phenomena being in the same position in the sky by pure coincidence would seem unlikely.

There's another issue; even if they are linked, the supervoid would only account for around 10 percent of the temperature decrease noted in the Cold Spot, experts say.

"The void itself I'm not so unhappy about," says Carlos Frenk, a cosmologist at the University of Durham in Great Britain. "It's like the Everest of voids — there has to be one that's bigger than the rest. But it doesn't explain the whole Cold Spot, which we're still in the dark about."

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