There's alot of rumors on the web and such as to who or what Salior Cosmos is...but um, I'll give you the answer...and you can trust me! ^_^
Anyhew, in the distant future, the evil, Chaos was causing trouble. Unable to defeat it, Sailor Cosmos grew scared and ran to the past in the form of a little girl.
That little girl was Chibi Chibi. Watching Eternal Sailor Moon loose agains Chaos, she realized what she must do, that she has to face her duty as Sailor Cosmos and reveals herself: the purest form of Sailor Moon.
In order to defeat the evil, she sacrifices herself and throws herself in the cauldron...Usagi-chan and all her friends are revived and all is good in the universe again. ^_^