The two bubbles could have been inflated by a past eruption from the supermassive black hole at the center of the Milky Way.

 The bubbles span 50,000 light-years across the sky.

They were found with NASA’s Fermi Gamma-Ray Telescope, which surveys the sky every three hours for the highest-energy light.

Among the 1,500 sources of gamma rays Fermi has mapped so far, nothing resembles the bubble-shaped structures, which stretch across more than half of the visible sky, from the constellation Virgo to the constellation Grus.

“We know it didn’t get to be that big by sitting there quietly all the time. It certainly has had big accretion events in the past, where material falls on it and then some of that material comes back out as high-energy particles blasted out in the form of a jet,” Finkbeiner said.

“You have to ask where could energy like that come from,” said astronomer Doug Finkbeiner, with the Harvard-Smithsonian Center for Astrophysics.

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