The Park hypothesis states that intelligent alien civilizations do exist, but they have not colonized the galaxy because they don’t want to.
"Since the only intelligent civilization that we know of is our own, our experiences may provide insights into how intelligent aliens behave. We regularly have a debate over what to send to space. It is known as the humans versus robots debate, or the manned versus unmanned debate, or with a more accurate description it could be called the both-humans-and-robots versus robots-only debate. Now, if an intelligent civilization such as ours is having this debate, then it is possible that intelligent alien civilizations are doing the same thing. Of course, they wouldn’t call it a humans versus robots debate. From their perspective it would be an our-species versus robots debate, and from our perspective it would be an aliens versus robots debate. And like our intellectuals, their intellectuals may conclude that alien spaceflight is obsolete, and a robots-only space policy would be sensible, logical, and right. They would dismiss colonization as a hopeless fantasy."