Startled astronomers said Tuesday they have discovered two massive bubbles of gamma-ray energy extending 25,000 light-years above and below the plane of the Milky Way galaxy like a squat hourglass.

“”They’re big, they’re sharp-edged and they contain a lot of energy,”” astrophysicist Douglas Finkbeiner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., said in a news conference. Finkbeiner led a team that used data from NASA’s two-year-old orbiting Fermi Gamma-ray Space Telescope to discover the bubbles hiding behind a background fog of gamma rays.

That fog occurs when particles moving at or near the speed of light interact with interstellar gas.

“”My first response when I saw these figures was ‘Wow!’ “” astronomer David Spergel of Princeton University said at the news conference. Spergel, who was not involved in the research, said “”we think we know a lot about our own galaxy”” and yet the bubbles, which are almost as big as the galaxy, were totally unexpected.

Researchers do not yet know what produced the bubbles, but the fact that they appear to have relatively sharp edges suggests that they were produced in a single event. Finkbeiner said that would have required the rapid release of energy equivalent to about 100,000 supernovae, or exploding stars.

One possibility is that there was a burst of star formation in the center of the galaxy producing massive, short-lived stars that exploded and ejected a great deal of gas and dust over a few millions of years.

Another possibility is that the black hole at the center of the galaxy — which has a mass about 4 million times that of the sun — shot out a stream of particles over a much shorter time scale, perhaps 10,000 to 100,000 years.

While there is no evidence that the black hole has such a jet today, it may have had one in the past. Similar jets have been observed in other galaxies. Starbursts have also been seen driving enormous gas outflows in other galaxies.

“”Whatever the energy source behind these huge bubbles may be, it is connected to many deep questions in astrophysics,”” Spergel said.

The report will be published Wednesday in The Astrophysical Journal.