Dark matter is defined to be any form of matter whose existence can be determined only through its gravitational interaction with visible matter. Dark matter is not visible because, by definition, it emits no detectable electromagnetic radiation by which to observe it. Studies of celestial mechanics involving the motion of galaxies imply that perhaps as much as 90% of the matter in a typical galaxy is dark. Astronomers speculate that dark matter may also exist in intergalactic space, and the resolution of this question may provide insight into questions regarding the ultimate fate of the universe.

Dark matter presumably reveals itself through its large-scale effects in the universe, especially in the formation of galaxies or galaxy clusters. Objects smaller than stellar clusters may not have detectable amounts of dark matter associated with them.

According to the laws of gravity, to exert a gravitational force on other celestial...