Class 9 nebula

From Memory Alpha, the free Star Trek reference

A class 9 nebula is a type of nebula, whose constituent compounds and elements have sensor-deceiving properties.

In an alternate timeline, the USS Voyager took refuge in one in order to make repairs, but corrosive gas from the nebula got onto Voyager when there was a malfunction in the ventilation system. Janeway decided to leave the nebula because she wanted to get back into open space to find allies and put together a fighting force to take on Annorax's Krenim weapon ship. (VOY: "Year of Hell, Part II")