The path to a civilisation moving out through the galaxy can be broken down into 4 steps: emergence of life, complex life, intelligent life, interstellar life. Regarding the first 3 steps, we know their probability is greater than zero. Regarding step 4, we know only that it's something no earthling species has done in the last 4 billion years. Not even us humans, though you think you can predict that we’re going to… Like it or not, the possibility exists that no form of life in the universe has ever expanded to another star, even if there are billions of life forms out there and some of them have reached levels of intelligence greater than ours. You make consider this to be one of the "highly unlikely possibilities", but here's why some of us disagree: Intelligent life in an untameable galaxy.