Well, it's not a magical alternate planes-slash-universes thing, cause I know what that looks like.

[Kravitz can travel through the planes and his tears in the fabric of reality look nothing like what the portal does.]

If it used to be alive, it might've been some kinda elemental or celestial, don't know exactly what happened to make it a past tense thing, but there was that whole war thing. Could've had something to do with it.