That s incredible. We re 26,000 light years from the galaxy s center, but these structures are so huge they cover a significant fraction of our sky. However, they emit no visible light we know of, so we can t see them by eye.
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They appear to be bubbles, thin-walled structures filled with hot gas. However, hot gas on its own can t make gamma rays. Most likely there are powerful shock waves as the gas plows into the thin material floating around just outside our galaxy, as well as strong magnetic fields locked in with the gas; together these can accelerate subatomic particles to incredibly high energies, making them capable of making the gamma rays Fermi saw.