Maybe you knew an art kid in high school. The kind of kid who spent most of his or her time drawing Big, Meaningful Art that was pretty good by high school standards, and whose Art usually expressed teen angst.
One such art kid produced Nibru:
Then the art kid decided to post Nibru on the Internet. Not to DeviantArt, the normal hugbox for teenage artists. No, the art kid posted Nibru at ConceptArt, an art site for aspiring professional artists.
Something about Nibru caught their attention, and soon some extremely talented illustrators (including guys who draw for Marvel Comics) were drawing their own versions of Nibru:
Go here for the whole Nibru story.