Signs of life, perhaps intelligent, detected inside an expanding protouniverse that a runabout accidentally dragged out of the Gamma Quadrant in 2370, forcing Benjamin Sisko to weigh its survival against the station's.
In the episode Playing God, Jadzia Dax and the Trill initiate Arjin return from the Gamma Quadrant with a strange mass of subspace matter caught on their runabout's nacelle. Placed in a containment field on Deep Space 9, it proves to be a tiny universe in the first stages of formation.
The protouniverse keeps expanding and threatens to destroy the station and eventually the Bajoran system. Kira Nerys urges that it be destroyed, but Dax's scans show patterns that suggest life, and possibly civilisation, developing inside it at an accelerated rate.
Sisko, unwilling to commit what might be genocide, decides to take it home. Dax and Arjin fly it back through the wormhole, with Arjin steering between dangerous verteron nodes, and leave it in the Gamma Quadrant. The lifeforms themselves are never seen or contacted.