Actually, online, when I discuss the idea, across forums, about aliens, I ask the same thing.
Prove the amazing evidence for such an amazing thing.
That’s it. Period. It just does not happen.
How many posters on these forums do you think actually possess the proof? The actual proof is secreted away. I use the "preponderance of the evidence" method to form my opinions. I also have faith in the aggregate of testimony given by many witnesses.
For example, have you actually seen an electron? You merely believe the empirical evidence that they exist. Have you seen Pluto? One again, you believe astronomers wouldn't lie or concoct their fuzzy photographs. It always comes down to the faith you have in the people who bring information to you. It NEVER depends on artifacts. Artifacts can always be faked.