Aliens are certainly rare in this world, but not quite that rare. Humans make up a good 70% of the population, but aliens are still widespread enough to be almost accepted. Imagine a black man in Japan, to use a modern example; he would be accpeted, but very obvious and would experience subtle discrimination in some places and well-meaning curiosity in others. Most alien planets have ether been wiped out by humanity or are out in the Unaligned Systems where the central human authorities have little power. Think of Tyra like a less diverse version of Tatooine; lots of aliens, but still a majority of humans.
As for intergalactic travel, I have plans for that; technology isn't at that level yet, bar perhaps using one hell of a generation ship.