Nibiru/"Planet X" doesn't exist, if it did exist there would be obvious evidence of its passage through the solar system and it would be easily visible.

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Girl: Mister Darwin! The stupid people are breeding and taking over the planet!
Charles Darwin: Tut tut, little girl, don't worry! I'll take care of them with my CHAINSAW OF NATURAL SELECTION! Ahahahahahhaha!!!!!!
-QUeen of Wands 12/08/2003