According to John Gribbin, those of us entertaining the idea that we might find intelligent life on other planets are destined to be disappointed. There appears to be no sign that another advanced civilisation (Gribbin typifies this as one which has advanced to the point of developing radio telescopes) exists.
His central argument hinges on Fermi’s remark that “If aliens exist they would be here“, not seeing any reported ‘alien sightings’ as credible. We have already started to explore beyond our home planet; it would be reasonable to expect another technically sophisticated civilisation (it would only take one) to have done the same. If they think Earth too ‘infected’ to visit they could have sent probes. If their politics are isolationist, it would have only taken one independent individual (with sufficient funds) to explore themselves (and would all alien civilisations be isolationist simultaneously?) We know that only part of our galaxy is inhabitable, they could have worked that out too, and reduced the time it would take to find us. One possibility exists, that we are the oldest and most advanced of the civilisations in the universe and the others have yet to reach a level of technical ability to explore space; but is this likely?
It’s not a happy thought for us members of the Star Trek generation, brought up with space exploration as just another one of those thing we did. We expected that we would have been on Mars long ago. We didn’t know that we wouldn’t have to worry about aliens because there weren’t any.