A proto-nebula
Proto-nebulae were newly-forming interstellar dust clouds.
In the 2360s, the USS Al-Batani became trapped inside a proto-nebula for three days by intense gravimetric forces. (VOY: "Bride of Chaotica!")
The USS Voyager crew witnessed the spontaneous emergence of a proto-nebula in 2375. It started growing very quickly and gave off plasma surges. (VOY: "Drone")