Sybaron

From Memory Alpha, the free Star Trek reference

Sybaron is a location in the Alpha or Beta Quadrants. The Seventh Fleet took heavy losses there in 2374, during the Dominion War. (DS9: "The Reckoning")

Like El-Gatark, it is unknown if Sybaron is a star, a star system, a planet or another type of astronomical phenomena. The encounter at Sybaron is mentioned frequently in the novel Hollow Men.