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How do we know we are not alone?
If you ask a scientist whether they believe in aliens you might be surprised by the answer. Scientists typically believe in extraterrestrial life - though out there, in space, not buzzing around our planet and scaring the locals. The Search for Extraterrestrial Life (SETI) is a respectable research area. What makes scientists sure that it's a worthwhile cause? Ref.
Search For Extraterrestrial Life - SETI (Hover)
I believe extraterrestrial life already exists in outer space, the only big difference is that it is usually micro-organisms that prove to not be significant enough to people besides scientists. I doubt any organization such as SETI will be able to find an organism that will be as smart than human beings.