Every galaxy you've ever seen in a photograph is hiding something. Beyond the glowing disc of stars and gas that the camera captures lies a vast, ghostly outer region called a halo, too faint to see easily but packed with clues about how that galaxy came to be. ESA has just formally committed to a mission designed to reveal those hidden haloes in unprecedented detail, and in doing so, finally answer one of the most fundamental questions in astronomy: how did galaxies like our own Milky Way form?
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