Again, you assume many things: - they are interested in expanding; - they didn't find a better solution to expand than going to other planets; - their materi

Again, you assume many things: - they are interested in expanding; - they didn't find a better solution to expand than going to other planets; - their material structure is similar to us (they are physical, they have the same size/speed/time perception as use) - the universe has no characteristics than we don't know about that could disallow us to perceive them (number of dimensions, or whatever) - they emit some signals among the one we can perceive; - they need and use energy as we do and as we understand it; - they are not actively hiding; - if we saw them we would realize what we saw. It's like an ant assuming that because they can't smell pheromones from another hill, there is no life there. Now, I'm for ocalm razor and I actually thing there is a strong probability that all those assumptions are true. But I don't know for sure that they are. They are just that: assumptions.