Why does the sky glow?
Astronomers have found that the sky glows in very energetic X-rays. They think the X-rays are the last gasp of material being swallowed by massive black holes. These objects hide behind thick walls of gas and dust, walls so thick that only radio waves and very high-energy X-rays can escape. Even moderately energetic X-rays are blocked.
When astronomers find massive black holes swallowing their surroundings, they can identify them by their peculiar behavior at optical and ultraviolet wavelengths and they call them active galactic nuclei or quasars. However, the massive black holes that bathe the sky in X-rays are too well hidden to be found this way, even though astronomers believe there are millions of them in the distant universe.
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