Published:
April 23, 2012

Photo by NASA's Goddard Space Flight Center
The only way to produce photons at energies much higher than about 1,000 MeV (which is solidly in the gamma-ray range) is to have protons moving at nearly the speed of light. Although we’re still not positive what causes the Milky Way’s bubbles, matter-antimatter collisions are not a possibility.
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