Judging by the way the stars bob up and down like horses on a carousel as they go around the plane of the galaxy, Oort calculated that there ought to be twice as much matter gravitationally propelling them as he could see. He postulated the presence of hidden "dark matter" to make up the difference and surmised that it must be concentrated in a disk to explain the stars' motions.
But credit for the discovery of dark matter—the invisible, unidentified stuff that comprises five-sixths of the universe's mass—usually goes to the Swiss-American astronomer Fritz Zwicky, who inferred its existence from the relative motions of galaxies in 1933. Oort is passed over on the grounds that he was trailing a false clue. By 2000, updated, Oort-style inventories of the Milky Way determined that its "missing" mass consists of faint stars, gas and dust, with no need for a dark disk. Eighty years of hints suggest that dark matter, whatever it is, forms spherical clouds called "halos" around galaxies.