If you take that route as an explanation then you can also say that maybe all humans on this planet aren't under the same rules. Why would they be?
Using a similar argument as yours, pretend you are a recently-converted Jew in the 1st century living in Palestine. To you, "the unknown" is not space, "the unknown" is North America and Australia. You have no idea those places exist, nor do you know if other intelligent life lives there. You, as a 1st century Christian, pose the question, "What if intelligent life lives outside of the discovered world?" (which is essentially what the OP said). Someone could respond with, "Maybe there is intelligent life elsewhere, its a really big world after all. And whose to say that beings elsewhere are even under the same rules?"
This makes God's law subjective and localized rather than objective and universal. If God's law does not apply to everyone in the universe, then I can just foolishly claim that I AM following God's law, but I'm just following his Law for the as-yet-undiscovered species on Zalgot-5 in the Andromeda galaxy.
I don't like the idea of God's law being subjective and localized; it opens up the door to moral relativism.