So this show is all about humans versus the machine race that they created. They've been to who knows how many parts of space, and had FTL for several thousand years. It is obvious that "life" can exist on other planets besides the colonies and Kobol, since we have seen it on New Caprica and algae planet, but in all the time and places they have been, they have never found another intellegent species.
Is there a significance to this? Would there be room in the universe for another alien race, or just humans?
Keep in mind that the original series had other species who built the robotic cylons. Maybe it could be a plot twist later in this last season?
I highly doubt it though.