Mine is probably the most boring: There are many intelligent species in the universe. Faster-than-light travel, however, really is simply impossible, meaning that there cannot exist a truly interstellar civilization. So while some species have probably settled solar systems other than their own through generation ships, suspended animation, time dilation, or whatever, their range of expansion is limited. This means that encounters between species of different planets are rare. Humans will most likely never contact any intelligent alien species, at most one or two. We might, however, discover evidence of their existence through telescopes or something.