The universe is so vast that it is impossible for the world to be the only planet with life. Of course, it's a little hard to give proof, but I believe we've contacted them. Maybe we are not, but people in the early days.
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How are the illustrations in the ancient caves or pyramids illustrating unusual formations? They must have seen them to draw them. People have an idea of how they look or their lifestyle, perhaps some may have experienced it in real life.
I know people can be so creative, but where do you think these ideas come from?
I strongly believe that they are somewhere in this vast universe. And for them, we're strange / unusual life forms. We ourselves, this enormous and scattered universe in other planets, other galaxies, etc. we are also strangers to life forms of living things.
We can't say we're the only ones in this infinite universe.