WASHINGTON (AFP) – Two huge, unexplained gamma ray emitting bubbles have been discovered at the center of the Milky Way galaxy, US astronomers said Tuesday.

Masked by a fog of gamma rays that appears throughout the sky, the bubbles form a feature spanning 50,000 light-years and could be the remnant of a supersized black hole eruption or the outflows from a burst of star formation, the astronomers said.

The structure spans more than half of the visible sky, from the constellation Virgo to the constellation Grus, and it may be millions of years old, the astronomers said in a paper The Astrophysical Journal has agreed to publish.

"What we see are two gamma ray emitting bubbles that extend 25,000 light-years north and south of the galactic center," said Doug Finkbeiner, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who first recognized the feature.

"We don't fully understand their nature or origin," said the expert who along with two graduate students made the discovery while processing publicly available data from NASA's Fermi Large Area Telescope, the most sensitive and highest-resolution gamma ray detector every launched.

Gamma rays are the highest-energy form of light.