In physics and fiction, a wormhole is a hypothetical topological feature of spacetime that would be, fundamentally, a 'shortcut' through space and time; although they are very popular in science fiction, there is no actual evidence that they exist. For a simple visual explanation of a wormhole, consider spacetime visualized as a two-dimensional (2-D) surface (see illustration, right). Now, if we 'fold' this surface along a (non-existant) 3rd dimension, it allows us to picture a wormhole 'bridge'. (Please note, though, that this image is merely a visualization, to allow the limited human brain to grasp an essentially unimaginable structure existing in 4 or more dimensions.) A wormhole has at least two 'mouths' that are connected via a 'throat', or tube. If the wormhole is traversable, then matter can 'travel' from one mouth to the other via the throat.
There is no observational evidence for wormholes, and, although wormholes are valid solutions in general relativity, this is only true if exotic matter can be used to stabilize them. Even if the wormhole is stabilized, the slightest fluctuation in space would collapse it. If such exotic matter, matter with negative mass, does not exist, all wormhole-containing solutions to Einstein's field equations are vacuum solutions, which require an impossible vacuum, free of all matter and energy.