The SETI project has not yielded any evidence of alien life
The Search for Extra-Terrestrial Intelligence has been operating for long without giving any evidence
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The SETI  project looks for alien life by listening to radio waves from space and discriminate from natural emission (form stars, galaxies,..) and signals generated by an intelligence. So far the research did not provide any results proving that detecting a communication is highly unlikely,
Looking for radio communication might be a limiting factor since it assumes aliens communicate in a method similar to ours. Charles Stuart Bowyer states that method might be ineffective  : it also requires that communication is directed towards earth or Solar System with a high-energy beam. Hearing a radio leakage similar to our radio and TV transmission is extremely unlikely.
Failure to detect a intelligence signal might imply that is not possible to communicate with external alien form of life.
Rejecting the premises
The "Fermi paradox"  states that Earth should have already been visited by alien or their probes.