According to Anti-Cosmo, it was created by the fairies centuries ago. The Anti-Fairies were stuck in Anti-Fairy World since it was created. Timmy wanted to talk to the Anti-Fairies, so he wished the door open, making the Anti-Fairies finally free. A prison was later created around the entrance.
Once Abracatraz was created, and possibly because they easily escaped with assistance, the Anti-Fairies were sent there instead.