Cosmic strings are hypothetical, bizarre, massive objects that may have formed shortly after the Big Bang; they can be thought of as tubular samples of the universe from about 10-35 second after the beginning of time. Cosmic strings are entirely distinct from the subatomic strings predicted by superstring theory. If cosmic strings exist at all they are predicted to be infinitesimally small in cross section but enormously long, perhaps forming loops that could encircle an entire galaxy. They would also be extremely massive – a one-meter length might weigh 1.6 times as much as as the Earth – and, consequently, give rise to intense and very strange gravitational fields. They provide one possible solution to the problem of dark matter in the Universe and have been posited as the seeds around which galaxies formed. It has also been suggested that they could serve as the basis for a type of reactionless interstellar drive; however, no observational support for their existence has yet been found.
1. Gott, J. R., III. "How pairs of cosmic strings can act as time machines." Physical Review Letters 66 (1991): 1126.