"De Sitter made observations in 1913 of double-star systems and became the first astronomer to prove that the velocity of light had nothing to do with the velocity of its source. ...De Sitter ...published three papers entitled "On Einstein's Theory of Gravitation and Its Astronomical Consequences. The first two... explained Einstein's theories, and the third proposed his own interpretation of the astronomical consequences... de Sitter and Einstein published a joint paper in which they proposed there may be in the universe large amounts of matter that does not emit light and consequently has not been detected. This became known as "dark matter"..."