OK, I think we provided a robust argument to explain why the galaxy isnt teeming with intelligent civilisations. People can argue we havent been looking long enough but the fact remains, if there were so many, we'd be falling over them.
But I digress. There are a number of other simialr scenarios which, for whatever reason, people like to believe are possible. Like the Kennedy assasination conspiracy, people seem to have no problem whatsoever in believing in them or, that one day they will become possible.
Take for example time travel. Well founded in sci fi. It happens oh so easily in Doctor Who, Star Trek and goodness knows how many other forms of the sci fi genre. Time travel isnt possible. It never will be possible. Why? Well if time travel is possible, where are all the 'time tourists'? Oh another they cant make themselves known argument.
But again, like the populuos galaxy, they would give themselves away. Intentional or not, they'd make mistakes. They would interfer just like an erant driver on a motorway may unintentionally cause a traffic jam by having a break down in the middle lane. The knock on effect would become apparent.
They wouldnt just travel to our time, they'd go further back. So how do we know our history hasnt been changed many times?
How, because we have consistent recollections of history handed down to us. We know that ancient Greeks didnt make the leap of discovering steam mechanics. We know the Roman Empire still fell. We know the Renaisance still happened. We know two world wars were raged.
If time travel was possible even way in the future, they havent come this way.
Of course there are theories that you cannot go back in time before the 'time machine' was made. But all you have to do to get around this, is find those other advance intelligent civilisations in the galaxy and 'borrow' their time machine that was built long in our past.
Even so, there are still no time tourists because time travel will never be possible.