A cosmic string is a spatial phenomenon characterized by its extremely narrow width and powerful gravitational fields. Cosmic strings are no wider than a proton yet have gravitational fields as powerful as those of a black hole. Cosmic strings can be detected by the presence of highly accelerated interstellar gases, which serve to outline the string and make it visible to standard sensors. A cosmic string emanates a characteristic set of subspace frequencies as atomic particles decay along its event horizon.
In 2367, the USS Enterprise-D was nearly destroyed after an encounter with a cosmic string fragment approximately 107 kilometers in length. The ship inadvertently became trapped by a field of two-dimensional lifeforms who lived in the string and were attempting to return home. In order to escape, the Enterprise used its parabolic dish to simulate the vibrations of the string long enough to confuse the lifeforms and disrupt their hold on the Enterprise. After the Enterprise was freed, the lifeforms continued on course to the string. (TNG: "The Loss")