The Hobus nebula was a massive stellar nebula in the late 24th and early 25th centuries. It was created in 2387 when the Hobus star went supernova, sending out a shockwave at multi-warp speeds, leading to the destruction of Romulus. Luckily, however, the star was collapsed by Ambassador Spock, using Red matter to create an artificial quantum singularity. (Star Trek: Countdown comic: "Countdown, Number Four")
The nebula continued to be a mystery for years. In 2408, in particular, Admiral Kathryn Janeway was interested in the nebula, believing that Species 8472 may have been responsible for the star's destruction in 2387. In 2408, she visited the nebula aboard the USS Tucker, hoping to find additional clues. During her time there, the ship encountered something unusual, but the nature of that encounter is not known, as Starfleet censored it from Jake Sisko's interview with Janeway. (STO novel: The Needs of the Many)