Another problem with assuming or arguing aliens could visit us or have visited us is that then you also have to explain why they're not here right now. I've never seen an alien and I haven't seen proof that they exist. Why would they keep their presence a secret? And if they visited us way back then, why did they leave again? One might think that in the universe, a planet supporting life is like a rich oasis in a desert. Life and interest would congregate around them because of the wealth of resources they provide.
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