The Oort Cloud is the diffuse spherical cloud of comets, asteroids, and dust that surrounds nearly all star systems, usually at a distance of several light-weeks to a bit over a light-year from the star. Named for a Dutch astronomer who popularlized the idea in the 1950's, the existence of such a distant source of comets and meteors was first hypothesized by a Lithuanian astronomer named Ernst Opik in 1932. The large number of uncharted bits of rock and ice make the Oort cloud a popular place for smugglers, pirates, and military scouts to ply their trade. Wildcat miners also prospect for water ice and other resources in the Oort cloud, especially in systems where entrenched corporate interests have sewn up all or most of the mining claims in more in-system asteroid belts. More heavily-defended systems have an Oort cloud seeded with mines, sensor drones, and stealthy warships to warn of any approach by snooping enemy vessels. The reduction in interstellar trade since the Holy Terran Empire shut down most of the jumpgate network has led to an increase in efforts to exploit the resources in many Oort clouds, to replace metals and ices formerly imported from elsewhere.
Oort clouds are not to be confused with the Kuiper Belt, which is a similar cloud of comets much closer in, and much more concentrated in a disc-shaped region along the Ecliptic.
Behind the Scenes
Of course, Oort clouds are real. The image is a public-domain NASA illustration we borrowed from Wikipedia.