In Star Trek, proto-nebulae are newly-forming interstellar dust clouds. The USS Voyager encountered a proto-nebula in 2375. It was growing very quickly and gave off plasma surges. (VOY: "Drone")

Proto-nebulae appear to be what is in real astrophysics to be the formation of nebulae from the gravitational collapse of gas in the interstellar medium or ISM. ISM is the gas and dust that pervade interstellar space: the matter that exists between the stars within a galaxy. The interstellar medium consists of an extremely dilute (by terrestrial standards) mixture of ions, atoms, molecules, larger dust grains, cosmic rays, and (galactic) magnetic fields. As the material collapses under its own weight, massive stars may form in the center, and their ultraviolet radiation ionises the surrounding gas, making it visible at optical wavelengths and turning it into plasma. An example of this type of nebula is the Rosette Nebula or the Pelican Nebula. The size of these nebulae, known as HII regions, varies depending on the size of the original cloud of gas, and the number of stars formed can vary too.