Considering the vast time scales and the vastness of the galaxy, the most plausible explanation for me is still that we happen to live in a historical dark zone.
But it's fun to speculate since any number or combination of events might be responsible for the absence of fellow civilizations.
There is a Great Filter argument that tries to say that we're really, really, really special or that our civilization-ending event might still be in the future. Since we're speculating, I don't think the biological filter argument is very plausible. Historically, we have always been proven utterly wrong after presuppositions of specialness. On the other hand if the Filter is ahead of us, it would have to happen very soon - good candidates for this remain AI, nano tech, or plain apathy. But then again assuming these would be an issue for every advanced civilization seems a stretch.
The simulation argument only makes a difference if the simulation is specifically targeted and limited to simulating us humans. If it's just a universe in a box, it wouldn't tip the scales either way.
The Drake equation and its cousins are nice, but they contain a lot of unknowns. Still, it's a good tool that shows how easy it would be to have statistical fluctuations resulting in a dark epoch.
I personally like the Agents of Suppression theory as well. It wouldn't even have to be another civilization suppressing everything directly - it could just as well be a completely automated armada of leftover killer robots. In my opinion it's one of the more plausible hostile alien invasion-stories as well: one day, we'll fulfill a set of unknown criteria and are simply tagged for extermination. But again, they would have to come our way pretty soon, like in the next 1000 years (which is not a lot of time). This would require a pangalactic drone network for observation and a fast local strike capability. Not completely unfeasible, but not exactly obvious either.
The absence of large-scale artifacts is also interesting, but we have to ask ourselves if we could even detect, say, a ring world or a Dyson sphere if it was in our direct neighborhood (and chances are, it wouldn't be that close). Also, intelligent civilizations would not necessarily produce them. However it would be enough if a few did build huge structures. Again, I don't think we're advanced enough to detect them (or their absence) with any confidence.
> the most plausible explanation for me is still that we happen to live in a historical dark zone.
I am no expert, but I remember reading somewhere that our Sun is among the first generation of solar systems that were born with enough heavy elements to make life based on carbon, oxygen, nitrogen, occasional iron etc. possible.
That there first needs to be some generations of starts going nova so that the stars have generated the heavier elements, and then the supernova explosions have to spread these around to that then the new interstellar clouds have enough of these element when they collapse to next generation stars and solar systems.
Another argument I heard, compatible with life being a fairly easy reaction;
A loglinear graph tracing the genetic complexity of life reaches zero about seven billion years. Obviously this implies an extraterrestrial origin of life theory. But it would also say life came into existence half-way into the big bang expansion and has been gaining complexity ever since. We are the result of this. But there's no reason to think there are other lifes that have that much of a head start. Thus it is not surprising that the manifestations of other intelligences haven't appeared.
It's a logarithmic graph. It doesn't go to zero. It just goes to 10^0 which is 1. And in this case one billion units of whatever. -1 is 100 million etc..
Yes and no. While our solar system is sufficiently metal rich, we're not especially early. There are systems of comparable generation that are younger than ours. This depends more on where you are in the galaxy than when you are.
But it's fun to speculate since any number or combination of events might be responsible for the absence of fellow civilizations.
There is a Great Filter argument that tries to say that we're really, really, really special or that our civilization-ending event might still be in the future. Since we're speculating, I don't think the biological filter argument is very plausible. Historically, we have always been proven utterly wrong after presuppositions of specialness. On the other hand if the Filter is ahead of us, it would have to happen very soon - good candidates for this remain AI, nano tech, or plain apathy. But then again assuming these would be an issue for every advanced civilization seems a stretch.
The simulation argument only makes a difference if the simulation is specifically targeted and limited to simulating us humans. If it's just a universe in a box, it wouldn't tip the scales either way.
The Drake equation and its cousins are nice, but they contain a lot of unknowns. Still, it's a good tool that shows how easy it would be to have statistical fluctuations resulting in a dark epoch.
I personally like the Agents of Suppression theory as well. It wouldn't even have to be another civilization suppressing everything directly - it could just as well be a completely automated armada of leftover killer robots. In my opinion it's one of the more plausible hostile alien invasion-stories as well: one day, we'll fulfill a set of unknown criteria and are simply tagged for extermination. But again, they would have to come our way pretty soon, like in the next 1000 years (which is not a lot of time). This would require a pangalactic drone network for observation and a fast local strike capability. Not completely unfeasible, but not exactly obvious either.
The absence of large-scale artifacts is also interesting, but we have to ask ourselves if we could even detect, say, a ring world or a Dyson sphere if it was in our direct neighborhood (and chances are, it wouldn't be that close). Also, intelligent civilizations would not necessarily produce them. However it would be enough if a few did build huge structures. Again, I don't think we're advanced enough to detect them (or their absence) with any confidence.