Its highly probable that, if human-scale intelligence/civilization exists on other planets, Von Neumann probes (or some similar, highly detectable signs of activity) would have reached us by now.
And they have not. This is an interesting paradox.
So first question: why even Von Neumann probes? What would be the purpose of a self-replicating spacecraft? If it's science, then there's no reason to assume we would even be able to detect it. If it's some kind of colonisation effort, it only requires more assumptions.
The basic assumption that the Fermi "Paradox" (I refuse to see it as a paradox) requires, is that technological civilisations remain unchanged over geologic time spans. That's a bold assumption, judging from our sample size of 1. Another assumption is that such civilisations grow exponentially for a practically unlimited amount of time (yeah, yeah, millions of years isn't exactly "unlimited", but on a human time scale it kind of is).
Given the vast distances in both time and space, it's much more reasonable to acknowledge that technological civilisations are simply too far apart (in both time and space) to make contact in most cases. A thriving local interstellar civilisation on the other side of the Milky Way within the Zone of Avoidance would be virtually undetectable by us, for example.
Actually I think the opposite assumption, that civilization radically changes over time, is an argument for it. Future humanity is not people on (sublight) space boats touring the stars like Star Trek, its seemingly some kind of unfathomable transcendent intelligence. Maybe its not even recognized as "civilization" by our standards.
And why would such an intelligence not explore its environment? What's the likelihood, if there are many such intelligences, that essentially all of these intelligences choose to remain static, stagnant and stealthy instead of making their presence known within their light horizon?
The exploring civilisation falls into a similar category: if they are super advanced, they wouldn't be seen unless they want to. So no data point to be had there either. If they're just advanced enough to cross the vastness of interstellar space, they'd take hundreds or thousands of years to get around, so very little chance for them to randomly bump into us at this specific point in time.
"Making your presence known" is also on shaky grounds, since it only takes one. By that I mean it only takes one locust-like civilisation that matches the "Grabby Aliens"-hypothesis to conquer the known universe within just a few billion years. This would imply, that it'd be highly likely that we ourselves would be part of- or descended from this super-civilisation. Again, the facts don't support that.
OTOH, it also only takes one civilisation that wants to prevent such scenario. They don't even have to be around anymore. All it takes is seeding the universe with self-replicating "sentinels" that simply sit dormant around M-, K-, and G-type stars and watch. Once they detect a space-faring civilisation popping up, they'd being their assessment. If the AI suspects a "Grabby-Aliens"-scenario, it's time for Exterminatus or some other, more subtle means of preventing the scenario from materialising.
So subtle in fact, that the affected civilisation might not even be aware of it. In fact, it could happen to us right now and we'd be none the wiser - ever wondered why it haven't made it back to the Moon in over 50 years ;)
The fun part about this hypothesis is that it's just another scenario that we wouldn't be able to detect or rule out: GRBs happen all the time, good thing Earth hasn't been in the way of a nearby one... Or imagine another Carrington Event, only 50x stronger, blasting us back into the stone-age - just another otherwise oddly calm G-type star having a little hiccup; nothing unusual our extra-terrestrial neighbours would think...
Basically the number of scenarios that would match our observations so far while still allowing for a universe teeming with technological civilisations, outnumbers any hypothesis that would truly present a "paradox" IMO. (just an opinion, not a fact, I could be very wrong on this)
This somewhat suggests or relies on humans not being uniquely territorial and resource hungry.
Kind of like the saying that if you see one cockroach you have many, many cockroaches.
1) Galaxies far away are also far back in time - they could all be Dyson-sphered by now and we would be able to know for millions or billions years.
2) This also might have happened elsewhere - we have no means of detecting what's going on in nearby galaxy clusters, or even within our own Milky Way really.
3) In this line of thinking, alien civilisations are always a walking oxymoron or paradox themselves: so they are super-advanced, yet have to be detectable, and they think like us but also don't, because they know about the consequences but don't care about them. They also act without reason but at the same time with purpose. And they are superlong lived (or their machines are) yet feel the need to breed at insane levels to settle entire galaxies because of... of the resources they actually don't need if they wouldn't expand like crazy? - but wait, you said there is no reason - ah my head's spinning :)
Or, how do we know they haven’t reached us? Maybe they stayed in the outer solar system. Maybe they came, looked, and left. Maybe they have some sort of cloaking device technology. Maybe they are really small. Maybe they came during the age of dinosaurs and probe is still out there in space, shut down and waiting to be discovered.
Unless human civlization collapses, why not?
> Or, how do we know they haven’t reached us? Maybe they stayed in the outer solar system. Maybe they came, looked, and left. Maybe they have some sort of cloaking device technology. Maybe they are really small.
Ifthere is even one more civilization sending out probes, that implies there are many within our light horizon sending out probes. I find it implausible that every single one would choose the same strategy of keeping probes stealthy and undetectable.
So I'm not sure why you'd think that a civilisation comparable to humans would be able to.
Also: if you can build Von Neumann probes, you can construct a Dyson swarm. You just instruct the probes to create the components of the swarm from the raw materials in your star system. But that would massively boost a civilisation's energy budget. Therefore, it's quite likely that any civilisation capable of building Von Neumann probes will have built a Dyson swarm.
So, secondly, it's about as paradoxical as the lack of detected Dyson swarms.
I mean, RNA/DNA has done that by trial and error alone. Of course the question is how do you keep it on task and not going extinct.
Here is one such video I highly recommend.
Second, life just seems not easy. Intelligent life doesn't necessarily have conditions to be sending out masses of timeless void travelling probes on and on. Probably a lot of the advanced ones struggle or lurch along as we do trying to subsist off the local ecospheres.
This is not how VN probes work....
VN probes work more like the lily pad riddle that humans commonly fail to understand.
>In a lake, there is a patch of lily pads. Every day, the patch doubles in size. If it takes 48 days for the patch to cover the entire lake, how long would it take for the patch to cover half of the lake?
The answer of course is the answer 47 to the riddle. With VN probes the answer would be "who the hell knows" at this point. Maybe interstellar space is a whole lot more dangerous than we expect and sending out countless self replicating probes isn't possible. Maybe the intelligent aliens out there are a whole lot smarter than us, and the idea of sending out probes that you effectively lose control of is recognized as a bad idea. Or, maybe there is no one else is out there. Hard to figure out with a sample size of one.