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Astronomers have used telescopes, operating at the visible wavelengths that we see with our eye, to reveal the true extent of the Milky Way and discover billions of remote galaxies that are rushing away from us in an expanding Universe. Each of these galaxies contains billions of stars wheeling around their massive central hub.Radio waves and X-rays, which lie beyond the range of visual perception, are used to detect a violent Universe, from exploding stars to gamma ray bursts and the Big Bang itself, with the associated discoveries of pulsars, black holes and quasars. Radio astronomers have also shown that the dark spaces between the stars contain vast tracts of cold hydrogen atoms and a host of molecules.