the problem wasn’t that astronomers didn’t know where the mass was. They did. It was in the halo—or at least in a “massive envelope,” the term that Faber and Gallagher adopted in an effort to be “neutral” as to the shape. The problem for astronomers was that they couldn’t see it. Not with their eyes, not with a traditional optical telescope, not with a telescope that could see in any wavelength of light. In which case, the mass wasn’t “missing” at all. It was just—to borrow the term that Zwicky had used in 1933—dunkle: dark. “Nobody ever told us that all matter radiated,” Vera Rubin liked to
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