Almost a century ago, Dutch astronomer Jacobus Kapteyn first proposed the existence of dark matter. He’d been studying the motion of stars in galaxies — a galaxy can be described, in rough terms, as a heap of stars, gas and dust rotating around a common center — and noticed that something was off. The stars in the outer layers of the galaxy were rotating much too fast to conform with the laws of gravity. Kapteyn’s hypothesis was that some invisible, massive stuff might be in and around the galaxy, making the outer stars reach the observed velocities. From the 1960s to the ’80s,…
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