Does aliens exist? Why don't we see alien civilization? Mr. Li talks about Fermi's paradox

Recently, a news hit the Internet where Canadian scientists claimed to have detected a repetitive signal of the fast radio burst FRB. Some take them as greeting from the extraterrestrial life and I was asked if extraterrestrial life form exists.
So, let's talk about an extraterrestrial life-related topic, the Fermi's paradox.
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(English) Subtitle maker : Yu Yingyao