This artist's rendering shows how a galaxy may build mass from narrow streams of cold gas. The European Space Agency's Herschel Space Observatory has observed that galaxies may grow more through this sort of slow and steady building, and not by collisions with other galaxies as had been previously thought. The rate of star formation peaked in the early years of the universe, about 10 billion years ago.
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Rebecca J. Rosen is a senior editor at The Atlantic, where she oversees the Family and Education sections.