Iso-star

Iso-star`s were rare spacial anomalies that existed in random and distant parts of the Alpha and Beta Quadrants. The star's were in actuality, small balls of energy (ranging from about the size of a basket-ball to the size of a car). 

The small star emanated Isolinium radiation, previously known for creating the illusion of duplicate life-forms of which happened to be engulfed in it. ("Pure Evil")

The star was tied to both the mirror universe and the original universe; where one existed in one spot, another identical star would exist in the same spot in the opposite universe. Whenever a starship would come into close range of the star, a simple scanning beam would activate the star's quantum properties. The next thing to happen was that an energy bolt from the star would latch onto the scanning starship, and transport the ship into the opposite universe: thus, the Iso-star was a gateway to the opposite universe. ("Secret Shuttles, Part I", "Mirrorlyness!")

People
Commander Seifer was obsessed with locating such star's, as he knew they would take him into the mirror universe - where he would complete his personal task of tracking down Lex Gotens and taking back his stolen symbiont. ("Avalon Battlefield, Part I")