People have proposed universes consisting of a single particle that goes between the beginning and end of the universe and back to the beginning again, only this time taking a different path so it appears to be another particle. That does not seem to correspond with our universe, which has a beginning but seems likely to expand forever.
Otherwise, you would seem to have the Creature from the Abyss of No Dimensions from Flatland. It knows nothing outside itself, but it is complex enough (somehow) to have an intelligence.
A less extreme example might be an intelligence that exists within another system with no external communication. The world telephone network is about as complex as the human brain, with a similar number of nodes, and a semi-random structure. It could be routing data for us, but also thinking. But what would it think about?