The Dark Side Cave, also known as the Cave of Evil, was a cave on Dagobah corrupted by dark side energies.
Hundreds of years before the Clone Wars, the Jedi Minch battled and killed a powerful Bpfasshi Dark Jedi leader, and his energies absorbed into its surroundings, first tainting the passage with the dark side.
During the Clone Wars, Yoda had a vision of the Dark Side Cave during one of his meditations. In this vision, Qui-Gon Jinn guided a nine-year-old Anakin Skywalker into the cave to face his fears, telling him the same thing Yoda eventually told Anakin's son, Luke, many years later.
Later, Yoda himself entered the cave under the guidance of Qui-Gon who had learned how to speak from beyond. Here Yoda had visions of the fall of the Jedi Order and of the rise of Darth Sidious.
During the Great Jedi Purge, Yoda realized that proximity to the Dark Side Cave could help neutralize his strong light side Force signature—thus preventing his detection by the Sith—and took up residence near the cave, living there for the next two decades, through the reign of the Galactic Empire.
Soon after the exile, an Imperial scout named Keog Boorn investigated a signal near the cave. A hidden Yoda saw how the scout left the place after remembering about the Bpfass Dark Jedi and hearing some terrifying sounds. Keog Boorn left the planet and reported that there was nothing in Dagobah, the signal just an atmospheric disturbance.
Yoda experiences a vision of Darth Sidious in the cave
During the birth of the Rebellion the clone of Starkiller spoke with Yoda briefly before entering the cave to look for the answers he sought. Inside he was unable to use his Force powers, except for the Jedi mind trick, which he utilized several times. He faced down other clones of Starkiller and had to carefully traverse some deep pits. He made it deep into the caves, and received a vision of the assault on the Salvation. Starkiller exited the cave, and left Yoda still sitting there and watching.
When Luke Skywalker traveled to Dagobah to train with Yoda in 3 ABY, the elder Master tasked him to enter the cave. Luke confronted an illusion of Darth Vader within and defeated it in a lightsaber duel, dipping into the passion of the dark side in the process. It was there that he realized what would happen if he turned to the dark side of the Force. Behind the mask of Darth Vader, he saw his own face. It also presaged the revelation that Vader was, unknown to him at the time, actually his father, Anakin Skywalker.
Luke re-visits the cave in 9 ABY.
Five years after Yoda's death, Luke Skywalker returned to the cave. This time, he had a vision of a possible past, one where Mara Jade accompanied Jabba the Hutt's retinue to the Sarlacc. She was able to pull Luke's lightsaber away from him, and Luke was pushed towards the Sarlacc when his vision suddenly ends. While in the cave, he found a beacon left behind by Jorj Car'das.
In 14 ABY, the dark side power of the cave was siphoned by the Disciples of Ragnos. This supposedly removed all of the dark energy, but it presumably returned to its source when the Scepter of Ragnos was destroyed or otherwise somehow regenerated itself. In 22 ABY, Anakin Solo, Tahiri Veila and Uldir Lochett entered the cave after Anakin had a recurring nightmare of becoming the infamous Darth Vader. There, Tahiri encountered a vision of her parents and grandfather, and Anakin faced the manifestations of his own doubts. The two both emerged victorious. Uldir, however, encountered nothing in the cave but vast emptiness.
Other Force-sensitive beings to enter the cave include Tash Arranda and Zak Arranda.