A mad scientist appeared in deep space with a starship whose drive didn't work. It broke the laws of physics, and as Scotty is so fond of saying, "ye canna do that". The USS Nia found the vessel and towed it back to El Nanth.
The owner of the ship one Dr. Jacob Burroughs Johnson had invented the Johnson continua drive, a mechanical means of getting from alternate universe to alternate universe. Here he had run into one where his hyperdrive didn't work. After some long discussion with Dr. Johnson and his team the Ane exchanged the Johnson continua drive for knowledge of the warp drive.
The All sat on the device for 23 years while its use was discussed. The Ane had been aware of alternate universes since, well forever. Math demanded they exist. Phenomena had demonstrated that they did. Here was a device that could test the limits of the AU phenomenon. However that said there was deep danger in even thinking about using it.
Then came the Ane Shadow War. The local Ane encountered Ane from an Alternate Universe. these Ane had gone down a totally different path. They used their psionic powers to control and manipulate like the Zhodani. An intense and invisible war broke out between the two branches of the All. The local Ane prevailed in their viewpoint. The are still involved in moving the Ane Empire to a point of racial self governance. (This was the source of all the unaccounted for ships in the back of Builder Station.)
It was decided that the alternate universes were not going to go away. Indeed they can come upon us. Ergo we cannot sweep the matter under the rug and ignore it.
The History Scouts were set up to deal with this. A branch of the ADF they work in secret, with most of the secret work being on the field end of the deal. Prime Directive: do no harm, and don't be seen. They travel to various AUs to do data work on why this or that might or might not have happened.
In two cases the History Scouts have been outed. What is called the Ashbyverse, an early 21st century Earth that did not experience the Eugenics wars. And the Lucasverse, a hyperdrive universe with a vast Galactic Empire.
In one case it was decided to settle into an existing world. Thindacarulle is a very different place with different rules of physics were "magic" works. The world has more Sentient species that have been encountered on one world ever, but the universe had no Ane.