aliens

Sometimes I wonder we are alone in this cosmos? is there a place where universe ended or what is after that place because if we see as human imagination a place can't ended fully. for eg. if we ponder earth is a universe so there must be beyond it if not then nothingness is what not discovered...

We all should know Kepler 425-b and Gliese 581d, and many more, but would do you think would be the best candidate? Planet? Moon? What do you think? My personal opinion is on Gliese 581 g.