A hypothetical one-dimensional (line-like) defect in the structure of spacetime produced in some models of the early Universe. If produced in the framework of a grand unified theory, such a string would be about 10−31 m thick, and have a mass of about 107 solar masses per light year. The strong gravitational effect of cosmic strings might have assisted in the formation of galaxies and of large-scale structure in the Universe. There is, however, no evidence yet for their existence. See also domain wall; magnetic monopole.