The Paulson Nebula was a dust cloud located in Federation space near the trade route between Zeta Alpha II and Sentinel Minor IV. The nebula contained swirling dust clouds, large rocks, and clumpy material which presented a navigational hazard for starships.
In 2366, the USS Enterprise-D used it to hide from a pursuing Borg cube. The Enterprise had to slow to impulse to enter it. Once inside, the nebula became too dense to go farther using impulse. The composition of the nebula cloud was determined to be 82% dilithium hydroxyls, magnesium, and chromium and found to be effective in screening the Enterprise from the sensors of the Borg, so the Enterprise continued on into this denser spiral cloud section of the nebula. The Borg did not follow them in but continued to scan for them and adapt to the situation. Eventually the Borg started sending in magnetometric guided charges and the Enterprise was forced to leave the nebula. (TNG: "The Best of Both Worlds")