In the far future, Sailor Moon has a more powerful incarnation called Sailor Cosmos.
As Sailor Cosmos, she faces a great and all powerful enemy, Sailor Chaos. Sailor Chaos in the Stars timeline of the BSSM story called simply Chaos. From Chaos came ALL of the evil villains Usagi had faces as Sailor Moon in the present timeline.
When Eternal Sailor Moon faces Chaos in the final book of the BSSM manga she is faced with a tough decision. Chaos has melded with the Galaxy Cauldron, the birthplace of all stars. She must decided if she will destroy it, along with Chaos and risk ending everything, since the birthplace would be destroyed as well. Destroying the Cauldron would destroy everything thus ending the battle forever.
While Eternal Sailor Moon is trying to make this choice, Chibi Chibi, a small mysterious girl that had come to support Usagi/Sailor Moon throughout the Stars series tells Eternal Sailor Moon that she MUST destroy it because there is no other way to end the fight. But Eternal Sailor Moon tells Chibi Chibi that she must have hope.
We find out that Chibi Chibi IS Sailor Cosmos, returned to her past self in disguise as Chibi Chibi. Sailor Cosmos had run from the lost battle with Sailor Chaos in the far future. Sailor Cosmos returned to the time when she as Eternal Sailor Moon COULD have destroyed Chaos and the Cauldron and ended it all. But Eternal Sailor Moon of the present will not give into that, and when she tells Chibi Chibi to believe she sparks new power and hope in Sailor Cosmos. Eternal Sailor Moon then throws herself into the Cauldron.
Inside the Cauldron, Sailor Moon finds all of her friends and a small sprite called the Guardian Cosmos (that is her proper name, not the ‘Cauldron Guardian‘ as she is sometimes called). Guardian Cosmos is there to protect this birthplace of stars and is the guardian of the Cosmos Seed. Guardian Cosmos offers Usagi and the others the chance to stay there as stars in their pure form, but Usagi politely refuses and asks that they all be allowed to live their lives as they are, together. Guardian Cosmos sends them all back to where they will safely arrive in the 30th century. As they are being whisked away, Usagi asks Guardian Cosmos what happened to Chaos. Guardian Cosmos tells her that Chaos has been melted away too small to be found, but may someday be born again.
From this we can only make the assumption that when Chaos is born again it takes the form of Sailor Chaos and then Usagi as Sailor Cosmos will be forced to battle it once again in the future.
The whole point of the Sailor Moon series is that light and dark cannot exist without each other. Usagi (the light) and Chaos (the dark) we both born from the same place (the Cauldron) and have and will both return there. They are constantly intertwined with one another. That is why Chibi Chibi wanted Eternal Sailor Moon to destroy the cauldron: if there is nothing left, there will be no more battles. But what Naoko is telling us through Usagi's actions is that everything is of the same spirit, and we are all from the same beginning... and we all return to the same end. It is a very powerful message.