> The thing is we know about natural phenomenon. We don't know whether aliens exist and are detectable. Occam's razor would favor existing natural explanations.

> The thing is we know about natural phenomenon. We don't know whether aliens exist and are detectable. Occam's razor would favor existing natural explanations. That's kind of my point though. We have tons of astronomical data, but except for neighboring systems it isn't precise enough to unambiguously detect an extraterrestrial civilization. So we go with the natural explanation for what we see, because Occam's razor would seem to suggest that. But by definition that means that if the universe has alien civilizations producing megastructures we would first assume those were natural phenomena. And then we would try to find explanations for these phenomena, and build models, which would be incorrect, but would provide a natural explanation for these megastructures. So, ironically, if the galaxy really was teaming with alien life and alien megastructures, we would likely just categorize it as natural phenomena, and wonder why we aren't seeing any signs of alien life. I agree that just stars and planets and such are very common and thus unlikely to be alien megastructures. And I don't think there are any objects we've documented that are unique in all the universe. Any potential candidate would have to be something relatively rare in the universe (but wouldn't have to be unique) and would have to be poorly understood (something where we haven't directly observed its formation, or it has properties that aren't explained by physical laws/phenomena that we've been able to validate in a laboratory setting) to be a candidate. As a stab in the dark, consider pulsars. Fairly few known in the galaxy, from reading the wikipedia page it seems like they have some particularly weird and poorly-understood properties. They rotate and emit radiation very precisely, precise enough you can sync atomic clocks to it. It's been proposed that you can use multiple pulsars as reference points to very accurately determine your location, like GPS (https://en.wikipedia.org/wiki/Pulsar-based_navigation https://en.wikipedia.org/wiki/Pulsar-based_navigation ). They emit huge amounts of radiation and are very small, meaning it would be much, much easier to extract energy from it Dyson-swarm-style. To be clear, I don't know much about astronomy, and I think that pulsars are almost certainly NOT alien megastructures, but I'm just pointing to it as an example.