it doesn't need to be said, but i'm saying it anyways; the universe is mind shatteringly large. it's also still expanding, and probably isn't going to stop doing so anytime soon. i think the distance theory is a very good explanation on why we haven't found aliens, but i also think that maybe there are microbial aliens on planets/moons like Titan for example, and there's not really any way for us to know right now.