The universe is contingent. This means that it could have been other than it was. At some stage prior to the existence of our universe, there was a potentiality for our universe and a potentiality for some other.

God is a being of pure actuality, with no potentials. As such, God does not require actualization by a prior being. God is also the creator of the universe - God selected one of the potential universes and actualized it, giving rise to the one we live in.

So my question is: In what being did the potential for universes reside, prior to creation? This potential cannot be in God, because God is pure actuality with no potentials. It cannot be in the actual universe, because then it would be prior to itself. So there must be a being external to both God and the universe, with the potential to become a universe.

Without a being of this type, there is no potentiality for any contingent thing - and thus God cannot actualize one. Therefore, since the universe does exist, the being-of-potentiality must be uncreated and co-eternal with God.

What is the nature of this being?

(Note: above uses of the word "prior" mean metaphysically prior, not previous in time. "Being" also means a thing that is, not necessarily a conscious life form.)