The recently proposed theory of "radical mundanity" offers a more subdued explanation: perhaps extraterrestrial civilizations are technologically advanced, but not to the "godlike" degree often imagined in science fiction. They may have hit a technological plateau only slightly beyond our own, finding the practical challenges of interstellar exploration and communication—which demand immense energy and time for potentially minimal reward—to be simply too tedious or impractical. Having explored their immediate cosmic neighborhood, they might have simply gotten bored and lost interest in further, prohibitively expensive galactic outreach.
