Sometimes when I'm laying in bed at night, I think about stuff like this:
Imagine our universe as it currently is, filled to capacity with other suns and planets bearing at least some kinds of life. Billions and billions of suns, and probably billions of planets. Rachet the number down to millions that have life, and possibly tens of thousands where life actually evolves into sentient beings.
The scenario is probably the same on each world: The smartest beings evolve, their intelligence leads them to make improvements in feeding and sheltering themselves. Populations increase, due to increased survival rates. As the population increases, demands on resources become strained and entire ecosystems are changed. Wars are possible, if fighting over resources is an option, but that's a story for another day. If they survive the fighting, then comes an inevitable reckoning.
Surely this has happened elsewhere, probably more times than you can number.
So what's next? The choice at some point becomes either limiting the population, or finding an off-world home for a significant portion of it. We, for example, have the capability of terraforming Mars into a habitable world over a period of about 1,000 years, planting greenery that will spread and, as it spreads, converts carbon dioxide into breathable oxygen and modifying the temperatures as it does.
But we can only seed it with human beings, not transport half the world's population there. We just won't have that capability in time to save everyone, given the way things are going.
And even if we could, so what? What happens when Mars gets crowded and resources run low there? There's no other decent planet in our solar system capable of supporting a big population of humans. And the distance to other stars is so, so great, that we have the same problem: We can, possibly, seed ourselves in other places, but not save all the creatures caught in the current over-population epidemic.
In other words, we are stuck, pretty much, right here. And so, I'll bet, are the other life-forms in this universe, far across the parsecs, grappling with similar issues. If only we could find a way to share information. It would be so much more useful than actually trying to get out there and conquer, as is our way.
Anyway, if I'm right, this probably means there are no cigar-shaped UFOs. No swamp gas, bearing silver spacecraft that rise from the mists. Whoever they are, our fellow Galacticans, they are probably as stuck as we are, and at some point they were faced with matching their populations to their resources and possibly, even stepping back from technology that was draining their planet dry of something they needed.
My guess is that there is life all over this big universe of ours. And that it's not filled with spaceships capable of making the jump to lightspeed or boldly going where no one has gone before.
My guess is that the most successful civilizations are the ones that have learned to manage their resources and control their populations. It's filled with farmers. And conservationists. And people who learned to use their resources wisely, and do without anything that was non-renewable.
Because the only other option is to perish. And, sadly, my guess is that for every population that succeeded out there beyond the Oort Clouds, there are a million that didn't, due to the exact problems we're facing now.
So we will see what happens here.