Robin Hanson and Nick Bostrom have this idea called the Great Filter, which is an attempt to explain the Fermi Paradox. I thought it would be nice to explain it as briefly as possible.
There are around 200,000,000,000 stars in our galaxy. Many of them have planets. So why hasn't intelligent life already evolved somewhere and spread throughout the galaxy? Why aren't the aliens already here?
Hanson and Bostrom's answer is that the evolution of space-faring civilisations faces some sort of inherent difficulty. Whatever this difficulty is, they call it the Great Filter. There are many possibilities as to what the Filter is. Perhaps most planets don't have the right chemistry for life. Or if they do, perhaps chemical replicators seldom come about. Or if they do, perhaps they seldom evolve into cellular life. Or if they do, perhaps they seldom evolve into complex life. And so on.
So, the Great Filter is whatever has so-far prevented any planet in our galaxy from evolving a galaxy-colonising civilisation. The basic idea is pretty simple; even obvious. Yet it has certain interesting implications...
The sinister conclusion drawn by Hanson and Bostrom is this: if we discover evidence of life on other planets (Mars is the usual example) then this is bad news, if we have aspirations of colonising the galaxy. Here's why:
Like all planets, the evolution on Earth of a space-faring civilisation is subject to the Great Filter (whatever it is). Perhaps the Filter is something in our past, such as the evolution of simple life. In that case, we can look to the future with some optimism, knowing that we have already cleared an enormous hurdle.
But suppose we found simple organisms (either living or extinct) on Mars. Then we could be almost certain that the Filter must be something that comes after the evolution of simple life. If we found multi-celled life on Mars, then the Filter must be something that comes after multi-celled life. And so on. The more advanced extraterrestrial life we discover, the later the Filter must be. Now the key insight: The later the Filter is, the more likely that it's still in our future.
If we're only concerned with our own future, then the absolute worst-case discovery would be radio signals from multiple alien intelligences at roughly our own technological level. That would be near-certain proof that civilisations at our own stage of development usually go extinct, since such a discovery would definitely place the Filter in our future. In that case, the Filter would occur during the transition from a technological civilisation like ours to a truly space-faring civilisation capable of expansion throughout the galaxy.
These conclusions might be more obvious with the use of some graphs I've created.
There could be multiple Great Filters (i.e. multiple huge difficulties that must be overcome before there can be a space-faring civilisation).
The argument is also complicated by the fact that, if someone else had taken over the galaxy, we might not be here to talk about it - since their activities might preclude our existence. However, if there wasn't a Filter, and if galaxy-colonising civilisations prevent other intelligent species from evolving, then intelligent life at our stage of technological development would only exist during an early stage of the galaxy. Only the Great Filter explains why we are here, all alone, so late in the life of the galaxy.
The only real objection to the Great Filter is that, even if multiple galaxy-colonising civilisations had evolved, we might somehow be unaware of them.
This page is simply a brief restatement of the ideas of others, with no contribution of my own.