Another solution is that Earth is the only place that life evolved in the Universe.
Another way at philosophically looking at, or for, alien is the Fermi Paradox.
In 1950, physicist Enrico Fermi – the creator of the world’s first nuclear reactor – came up with a paradox which says due to the age and size of the universe there is bound to be a civilisation much more advanced than ours, which begs the question: “Why is it just us?”
The solution, many scientists argue, is once a civilisation reaches a certain size, it eventually kills itself off, either through war with advanced weapons or natural disaster.
Some experts, however, have previously warned against trying to contact any intelligent species.
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