Amphion

Amphion was a star in the antimatter universe, and occupied the same space as the Beta Niobe supernova in the normal universe.

While Beta Niobe was dying, Amphion was just beginning its life as a star. The combination of a dying star and star being born in the two universes, as well as occupying the same space, served as a gateway and allowed the USS Enterprise (NCC-1701) to cross over into the antimatter universe. It had also allowed Karla Five to cross from the antimatter universe into the normal universe. (TAS episode: "The Counter-Clock Incident")