Shadow matter is a hypothetical form of matter that is thought to interact with ordinary matter only through the force of gravity. If shadow matter exists, it is a form of dark matter. Dark matter is invisible matter that scientists believe makes up most of the mass of the universe. Dark matter is invisible because it does not give off, reflect, or absorb light. Unlike other hypothetical forms of dark matter, shadow matter cannot be detected in any laboratory experiment (see Dark matter ).
Theoretical studies have shown that if shadow matter does exist, its properties are very different from those of ordinary matter. Because of these differences, such objects as stars and planets made of shadow matter could not form.
The possible existence of shadow matter has been proposed in superstring theories. Physicists believe that such theories may explain the nature of all particles in the universe and of the forces that control the behavior of the particles. Superstring theories suggest that subatomic particles, including particles that transmit forces, are actually extremely short, one-dimensional strings. Theoretically, the strings are so short that they appear to be points. Until recently, scientists had assumed that subatomic particles formed tiny points.
