Planet Watcher to Bill Johnson
Battlestar Galactica is a Sci-Fi genre based on a reality where there pretty much is no sentient life except for humans and cylons (which are robots made by humans).
People who are centered on believing there are no aliens in space tend to be subscribers of the Battlestar Galactica material.
The problem with this perception of reality is that it places a special emphasis on life and intelligent, which contradicts the very nature of life it self.
If we look at life on Earth and what we know from this universe as of right now.
We are look at a galaxy with over 200 billion stars, over 600 million Earth like planets, I mean definitely Earth like (not habitable desert, arctic, or Titan like planets/moons, in which the number would be much higher). Out of those 600 million Earths, the probability of one developing intelligent life corresponds with the age of the planet, its degree of turbulence (the less the better the chances are) and its corresponding resources within the same system and nearby systems.
The most conservative estimates puts the potentiality of intelligent life (human level) at 780 thousand and (beyond human level) 403 thousand.
So even if you put the numbers as low as you can without ignoring certain scientific factors, you will still end up with 403,000 sentient species that are traveling the galaxy.
Your question should be, how come none of them ended up here? Correct? 
