“Playing God” is episode seventeen of season two of Star Trek: Deep Space Nine.
Another Trill, one in the programme to get a symbiont, but currently unjoined, is visiting DS9 for Dax to evaluate. Jadzia’s predecessor with Dax, Curzon, had the reputation of having failed more Trill in the programme than any other – which originally included Jadzia herself – so the new applicant isn’t looking forward to it.
Whilst they are on a trip to the Gamma Quadrant in a runabout, they get entangled in something, and bring it back to the station. It’s first described as being a sort of space seaweed.
When they evaluate it further, Dax says what it actually is is a proto-universe that they’ve accidentally brought into their own. This is a bad thing, as the proto-universe is expanding. As it does, it’s displacing the current universe. It has the possibility of eventually destroying the current universe with itself, if they don’t manage to get it back where it belongs.