After a life long interest in astronomy, science, aviation and space exploration. and in all that time having seen two extrarerrestrial vehicles, there is no room for doubt that in my mind that intelligent life exists elswhere in the universe. I say extraterrestrial vehicles rather than UFO's because that is what they were. Having done a bit of flying myself, you sort of know the why's and wherefore's how terrestrial technology keeps things in the air. These were something different, and using a technology, which if we had, it we would be using to. As if that was not enough in the way of proof for me, my 8 year daughter, to whom, once having asked me, "What's that dad?" (It was a DC3) I replied, "It's an old Dacota" everything flying, including the fast jets form a near by air base at the time, became an 'Old Dacota!' It was she that called me from the house and asked "What is it?" recognising that this was indeed something different and definately not an old Dacota. It was infact a classic Saucer shape with one exception, that being that the mid upper section was a series of panels. As the whole thing was a mat black ( no lights bells and whistles) you could only just make out that feature as, rather like when you brush a piece of velvet one way, then the other, there was a feint sheen you could just make out as they ticked round at about once a second.
Scientists rightly keep a healthy scepticism about these things, and demand absolute proof, and for sure there are many false and bogus reports of sightings. I guess it takes such sightings in the plain light of day that are quite unambiguous, to make you a believer. Naturally one asks the question, "Why don't they contact us?" The answer to that is I believe, that if they are smart enough to travel interstellar distances, they are smart enough to figure out that apart from a few intellectual bright spots, this is still a violent, socially, politically and technologically backward planet. Not quite the sort you would want to invite home to meet mum and take tea with yet, and just the sort that would misuse and abuse your technology.
As for how the do it, I think many scientists reject the idea of interstellar travel because of the paradigms created by their current knowledge of physics. I am in the camp that says that given our present state of knowledge, we don't know how to do it yet! I always like to sight the head of the German patent office in 1904, who on hearing of the Wright brothers first flight declared,"Everything that can be invented, has been invented!" "Yer right!" The fact of the matter is, that there are many new branches of science and technology, yet to be discovered. I just hope we have the social, political and moral maturity to use them properly.
If it was possible for the building locks of life to form intelligent life on earth, then one day we may be able to find life in another planet but what we may term intelligent on earth may look primitive when such life is found. If it takes billions of years to site such life, to our own day intelligence, it does nut exist.