Another question not yet asked in all this is why Earth? Given the difficulties discussed in even conceiving inter-stellar travel at all it seems totally impossible that Earth with its evolved life could have been discovered by chance while these aliens were cruising around investigating solar systems in this part of our galaxy. No they would have need some reason to come to Earth specifically. Problematic in that it is only in the last 100 years or so that man has developed and used technologies that would allow aliens from elsewhere in the galaxy to become aware of our existence.

In so far as their being life, even intelligent life, elsewhere in the universe. Given the vast number of stars in a galaxy and the vast number of galaxies in the universe I would say it is almost certain. The plausibility of a visit from one of these other intelligent life forms in a whole other matter. Not unless one expands the the meaning of a visit to included mental contact through some as yet unknown psychic process. I not saying I believe in this either, I just think it more plausible than little green men in space ships arriving.