Prehistory

Dark matter, in an astronomical sense, is introduced to explain the difference between how objects in the sky ought to move, according to some preconceived notion, and how they are actually observed to move. In that sense, the first application of this concept was employed, as one might expect, by the Greeks. In the fourth century BC Eudoxus of Cnidus, a student of Plato, proposed that the stars and the Sun were attached to transparent spheres with the Earth at the center. The spheres rotated about the Earth: the outer “stellar” sphere once per day, and the inner “solar” sphere also once per day with an additional annual motion with respect to the stellar sphere. The spheres were not actually dark but transparent, so that we could see the stellar sphere through the solar sphere. The spheres did not possess the attribute of mass – or weight – so perhaps it is not proper to speak of them as matter. But they were certainly unseen constructs proposed to explain the observations. The most famous student of Plato, Aristotle, increased the number of spheres to 55 to account for planetary motion, and he seemed to believe in the actual reality of these spheres.

The system was codified by Ptolemy in the second century AD and reigned supreme for about 1300 years. By the time of Copernicus the entire construction had become very complex with spheres on spheres and off-center spheres in order to explain the increasingly precise observations of planetary motions.