A wormhole is a hypothetical "tube" in space connecting widely separated positions in the universe. Eric Weisstein's World of Astronomy Wormhole solutions to Einstein's general relativity field equation exist and were known already in 1916. All wormholes require exotic material (material with negative energy density) in order to hold them open. However, in 1993, M. Visser showed that quantum wormholes, once formed, are stable.
References
Halpern, P. Cosmic Wormholes: The Search for Interstellar Shortcuts. New York: Dutton, 1992.
Visser, M. Lorentzian Wormholes: From Einstein to Hawking. AIP, 1995.
Weisstein, E. W. "Books about Wormholes." http://www.ericweisstein.com/encyclopedias/books/Wormholes.html.
