Is Nibiru real or fiction?
The wikipedia page on Nibiru covers most of the insanity (and I mean that literally) pretty well.
It is not real. There is no rogue planet out there waiting to cause havoc on Earth. It hasn't been there for the last 20-some-odd years that people have been talking about it.
It started in the mid-90s with a woman named Nancy Lieder who claimed to be receiving telepathic communications from a race of aliens called Zetans (so named due to being from or near Zeta Reticuli). She started a web site named ZetaTalk (not linking it because you don't want to go there) to relay these communications, and to host discussions on the fringiest of fringe theories (ancient aliens, chemtrails, and general X Files stuff).
Nancy herself only called it Planet X - others took the name Nibiru from the writings of Zecharia Sitchin, who believed Earth was visited by aliens thousands of years ago based on writings from ancient Sumer.
A member of the ZetaTalk community, Mark Hazelwood, wrote a book on Planet X/Nibiru titled Blindsided: Planet X Passes in 2003. From there it's passed into the general conspiracy theory zeitgeist, and is used by the black helicopter/"9/11 was an inside job" crowd as another talking point in the whole "the gummint is hiding things from us" discussion.
Yes, the government is hiding some (genuinely classified) things from us. Nibiru isn't one of those things. It couldn't hide Nibiru from us if it wanted to. Too many people own telescopes and watch the skies - if a rogue planet were truly making its way through the solar system to menace Earth, we'd know about it.
More importantly, we'd have known about it in 2003 when it was first predicted to appear. It got a bit of a revival as 2012 approached, which was supposed to be the end of the world based on the end of the Mayan Calendar (because everybody's car explodes when the odometer ticks back over to 000000).
Obviously, the world didn't end in in 2003 or 2012, and it's not going to end due to a rogue planet that nobody can see. But, since the best evidence of a conspiracy is a complete lack of evidence for a conspiracy, people still claim it's out there.
They are wrong.