Aliens, in the sense of life not from/on earth, is pretty much certain to exist. There are an unfathomable amount of planets out there, and life arose here on earth pretty much as soon as the conditions were favourable, considered on a geological time scale. That's been my opinion for most of my adult life, and getting a biology degree has only reinforced it. I find it hard to believe that nowhere in this staggeringly enormous universe were the conditions right again. What isn't hard to believe is that humanity will never encounter alien life due to distance/advancement.