If infinite power isn't required to bring about the universe, why cannot our universe be the only act that a God could perform?

But is the universe truly isolated?

It is a reasonable creation philosophy that the action (whether directed or not) was not "creation" but "division". That a nothingness was divided into positive and negative particles and energies all of which balances to a sum of nothing again. Perhaps in the long term it will again collapse into nothingness and perhaps it has already gone through this cycle through infinity.

We invented gods to explain diseases, weather events, and a lot of things we couldn't explain back then. Our knowledge has advanced, so our explanations with time had been less "lets suppose an almighty god did it" and more "this happened because...". The origin of the universe still is in the lets-explain-it-with-a-god category, just because we don't figured yet an explanation. And yes, maybe there was a god behind it, but as turned that we didn't need a god to explain the thunder, maybe we won't need one to explain the origin of the universe, or maybe we won't ever be able to explain it. But putting as explanation another unknown mistery that is far bigger and more complex to happen than the current universe seem to be a step in the wrong direction.

Seriously though, people should stop trying to go so far beyond when we can barely get out of our solar system. Let's worry about starting to explore the universe, colonizing other planets and just then try to answers such questions.