One technique I sometimes like for giving in-world explanations when you still want to leave things open is to actually have a couple of conflicting viewpoints. So, researcher SmartWolf writes, “the Rift is a random circumstance!” and researcher CleverFox writes, “the Rift is a sentient godlike entity!” Maybe they even argue a bit, like scholars sometimes do, and call each other wrong in their texts. “…while CleverFox claims there is a reason for us to be here, I consider this mere childish fantasy that has no place is serious scientific endeavor…”
It can help make it clear what’s definitely true, because they agree on it - there’s a Rift! It gathers people! - while leaving some things more open to interpretation or further IC investigation and theorizing.
Of course… this also means more work in terms of creating those books and those personas of various authors. Maybe it’s something you could do via roleplaying, or just by asking some other people to write their own book-snippets and collecting them together?