NASA's Fermi Gamma-ray Space Telescope has discovered a massive structure spreading halfway across the Milky Way.
It may, says NASA, be the remains of an eruption from a supersized black hole at the center of our galaxy.
"What we see are two gamma-ray-emitting bubbles that extend 25,000 light-years north and south of the galactic center," said the phenomenon's discoverer, Doug Finkbeiner, of the Harvard-Smithsonian Center for Astrophysics. "We don't fully understand their nature or origin."
The structure spans more than half the visible sky, from the constellation Virgo to the constellation Grus, and may be millions of years old.
Finkbeiner and his team discovered the bubbles by processing data from Fermi's Large Area Telescope (LAT).
While there had been hints of the bubbles' presence in earlier data, they hadn't been detected partly because of a fog of gamma rays throughout the sky. But by estimating the extent of the fog, the team was able to isolate it from the LAT data and reveal the giant bubbles.
Scientists now are conducting more analyses to better understand how the never-before-seen structure was formed. Its shape and emissions suggest it was created as a result of a large and relatively rapid energy release.
One possibility includes a particle jet from the supermassive black hole at the galactic center. Fast particle jets powered by matter falling toward a central black hole have been observed in may other galaxies. While there is no evidence that the Milky Way's black hole has such a jet today, it may have had one in the past.
The bubbles might also have formed as a result of gas outflows from a burst of star formation, perhaps the one that produced many massive star clusters in the Milky Way's center several million years ago.
"In other galaxies, we see that starbursts can drive enormous gas outflows," said David Spergel of Princeton University. "Whatever the energy source behind these huge bubbles may be, it is connected to many deep questions in astrophysics."