I used to watch Ancient Aliens and at the time bought a lot of what it said. I remember it claiming the Ark of the Covenant was a nuclear-powered machine that fed the ancient Israelites manna (don’t ask me). It went on to posit “where else could they have gotten that other than aliens?” Now, even taking that rather ludicrous claim at face value, there’s still a problem. It’s bad logic to claim that whatever you’re saying must be true just because there’s no other thing to disprove it.
Embarrassingly, I ate that claim up. I have a friend in a similar boat. He has told me about having seen lights in the sky that he attributes to aliens. Most of his evidence seems to be that it couldn’t have been anything else. It didn’t make helicopter noises, turns too much to be a plane, and is too bright to be a drone. The problem with this reasoning is that all that means is we don’t know what it is. Until you prove that everything else it could ever be is wrong, it isn’t aliens by default (never mind that we have no idea of all the things it could be).
Watch out when someone tries to prove something and then simply starts trying to disprove any other explanation. All that means is that we don’t know what the explanation is. I’ve found conspiracy theorists to be very bad about this: tThe World Trade Center attacks were an inside job because the official reports aren’t true; JFK was assassinated by [insert nefarious thing here] because Lee Harvey Oswald couldn’t have been the shooter and [insert nefarious thing here] would want him dead. Unless their claim is simply that the official reports are wrong, this is a fallacy.
The core of the problem is that they’re basically saying that if you can’t disprove an answer (with other explanations they’ve discredited), you can’t say that it isn’t true. But you don’t need to say that you know their positions are wrong, only that they aren’t true yet. They haven’t provided evidence for their claim, and you should respond with a hearty .
Remember, even if there is no other explanation, your idea stands or falls on its own merits. It starts out debunked, and you need to re-bunk it.