What if aliens are more of a psychiatric phenomena? Aliens have the ability to cross galaxies and stay undetectable to most of our instruments. They manipulate our senses to stay hidden to us, and have the ability to wipe our memory, which only gets better over time as they interact and experiment on us more. The amount of technology that an alien civilization amongst us has would be incomprehensible to us, using aspects of physics that we have no idea exist. Probably they are observing us now in detail, waiting to reveal at the correct moment - or more likely, they are revealed when we discover far more advanced technology and we learn how to see them.
OK, thanks for reading.