The Galaxy Cauldron was the birthplace of all stars and all living things in the galaxy. It was first mentioned in Act 47 of the manga and was stated to be at the center of the Milky Way galaxy, in Sagittarius Zero Star. It is the most powerful star in the universe.
Everything is created and destroyed within the Galaxy Cauldron - life is created there, born as tiny stars that eventually grow into planets and the guardians who protect those planets, the Sailor Guardians. Sailor Guardians who die can be reborn there, or they can choose to stay in their purest forms. Galaxia said that it is "the holy land of the stars, The Galaxy Cauldron. Strong stars and weak stars. Trash and Sailor Guardians. This is the place where all the stars throughout the galaxy are born." The Galaxy Cauldron looks like a bubbling stream of energy just below a cliff.
In the final manga arc, the Cauldron was taken over by Chaos, the purest form of evil, and Usagi was faced with a choice: destroy the Cauldron, along with Chaos, thus ending all war but also all life, or preserving the Cauldron, ensuring Chaos would return someday, but preserving all life as well. Chibi Chibi tried to convince Sailor Moon that destroying it to end all suffering was the right thing to do, but Sailor Moon told Chibi Chibi not to give up, to always have hope, and that living life was more important. Sailor Moon then threw herself into the Cauldron, using the Silver Crystal to drag Chaos down with her, dissolving the dark entity.
Guardian Cosmos, the small but powerful guardian of the Cosmos Crystal, resided within the Galaxy Cauldron. When she saw Usagi in the Cauldron, she was surprised that Usagi and her friends could retain their physical forms within the Cauldron. She asked Usagi if she had come to the Cauldron to throw her life into it and start a new star's history, or if she wanted to live as she was. Given the choice, Usagi said that she wanted to live her life as she was, no matter how difficult it might be, to build a future together. Guardian Cosmos granted their wish, and as they were being pulled away, Usagi asked about Chaos. Guardian Cosmos told them that Guardian Chaos and the Chaos Seed had been dissolved into the Cauldron, but could be born again.