This question has been confusing humen beings for over thousands years. Maybe Newton, Einstein and Tesla also didin't give the clear answers throughout their whole life. When the Curiosity mars rover the first time landing on mars, people all around the world expected to meet new "friends" from another world, but unfortunately, it's a silent world without any creatures. I don't know the purpose of people who persevere to look for aliens. especially those sci-fic movies give us enough imaginations about aliens. But most of them are carbon-based creatures, but what if they are silica-based creatures, can we find them? So, this is a boring question. it makes no sense to research it. If they exist, they must live thousands lightyears aways that neither we nor they can reach the others' planet. If they don't exist, we are the only creatures in the universe, we must have our own speical duty to finish. it's not necessary to spend time on searching them.