A subspace interphase pocket is a self-contained area of subspace which can cause stability loss if impacted. Alternate universes could exist in these pockets.
In 2370, Jadzia Dax and Arjin, a Trill initiate who had been assigned Dax as his field docent, encountered a subspace interphase pocket while piloting the runabout USS Mekong in the Gamma Quadrant. After recovering from the impact, they discovered they had picked up an unknown mass, which Dax termed "subspace seaweed," which had become embedded in the runabout's starboard nacelle. After bringing the mass back to Deep Space 9, it was discovered that it was, in fact, a protouniverse undergoing an accelerated development. As the growth of the protouniverse was threatening the station, Dax and Arjin were forced to transport it back to the Gamma Quadrant, where they deposited it back in the subspace pocket. (DS9: "Playing God")