The Black Gate is a 21-dimensional space/time/thought discontinuity located in the New Canada system. The aperture is always open, but is invisible (hence the name). The destination of the wormhole is determined by the orbital positions of the three Trojan planetoids around it, and its position in relation to the sun at the moment of entry. (Star Trek: Banshee Squadron)
The Black Gate is closely orbited by the three Trojan planetoids, Nanuq, Amaguq, and Tekkeitsertok, and as a group they all orbit the New Canada sun in the trailing Trojan position behind the fourth planet, Txamsem. While the L5 Lagrangian position behind the gas giant is a natural gravitational collecting point, the arrangement as a whole is not a stable one, and it is not understood how they maintain their equilibrium without either falling into the sun or Txamsem, or spiraling out of the system altogether. It is possible the Black Gate has some unknown effect on the orbits of the Trojans, holding them in place.
The Black Gate was discovered in July of 2383 when Banshee Squadron (VSF-6501) accidentally fell through it while fighting a Breen scout ship. At the time, the wormhole destination was near the planet Kurnugi, over 1.6 billion light years from the Milky Way galaxy.