A tiny, rapidly expanding embryonic universe brought back to Deep Space 9 from the Gamma Quadrant by accident, which threatened to displace the station and Bajor as it grew in Playing God.
The protouniverse is a universe in the earliest stage of its growth. It is discovered in the Deep Space Nine episode Playing God in 2370, when Jadzia Dax and the Trill initiate Arjin, whom she is assessing for joining, return through the wormhole with a mass of what looks like subspace interference snagged on a nacelle.
Placed in a containment field in the science laboratory, the object begins to expand, tearing at the station. Dax then finds signs that there may be life, perhaps intelligent life, within it. Destroying it could mean wiping out a civilisation, while leaving it would in time destroy the Bajoran system.
Sisko decides to return it to where it was found. Arjin pilots the runabout back through the wormhole, steering between the verteron nodes that could rupture the containment field, and the protouniverse is left to grow in the Gamma Quadrant.