The Initiative discovered this transcendent realm, and the powerful beings that reside there, as the result of a complex magical ritual performed to locate a missing person. This is a place that exists like a web around the Verses, without being attached to any particular one. It is in between quantifiable reality, yet intrinsic to it.
This space is where the Four Evers reside, each having shaped part of it as their own domain, devoted to four aspects of sentient being. Phia, the mentor to Para and first person in Synthex to understand portals, was flung into the In Between after a lab accident. The Four Evers seem to be eternal and implied that there was more to the portals than just a more connected multiverse.
Introduced in #39 Eyes of the Wizened.