Well, the short story is that this isn't the first world to have imPorts. In AD 2008, imPorts first started appearing in 'The City,' a nameless, essentially lawless city in roughly the same location as this world's New York. They eventually learned that they were brought there by Lachesis, one of the Fates from Greek mythology, as part of a war with her sister Atropos. Atropos destroyed the original universe, but Lachesis escaped with many of the imPorts into this world in approximately AD 2014. [He says his years digit by digit, out of habit-- two zero zero eight, two zero one four.] The history is a little hard to follow, but essentially Lachesis rewrote reality in this universe to create the current Porter, which continues to bring people from different universes in and out of this world, seemingly at random. Lachesis herself is apparently in hiding. A very small number of people remain in this universe who remember the City, but they've passed the information down as much as they can. Recently, we've had reason to suspect that Atropos has surfaced and begun influencing events in this world, so people are more aware of the history than they might have been.

...It wasn't much of a speech, I'm sorry. [Again with these apologies... he attaches a link to the post.] I've only been in this world since March of this year, so this is all second hand. Kaneki-- you're speaking to him elsewhere on this post-- takes notes about what imPorts have discovered in this world and what was carried over from the last one, but they're very detailed.