in theory, the chance that we are alone in the universe seems incomprehensible. in the other side of the same theory is the fermi paradox - given the scale and time they would have had to do so, the chances that if their were aliens they wouldn't already be swarming the entire galaxy all around us, are also incomprehensible.
simply put, our galaxy is both small enough and old enough for many many species of aliens to have already being everywhere a good few million times over, so the fermi paradox asks, "where the fuck are they?"
unfortunately, most possible explanations to this are lost in the world of probability, in the sense that they demand making a claim about the nature of aliens, and the answer will always be, "out of the millions of alien civilizations that could have arisen, not a single one of them breaks that rule?"
for example, you might say that the aliens have something that looks like the prime directive, and are treating us like a zoo for observation. again, "all of them? every single alien species agreed to this?".
you might say that it takes too much resources to colonize other solar systems, and some might not want too. and again, "not a single one of them? millions of potential species, and not one of them had the aspiration and took the risks of doing so?".
the same pattern goes on with almost every single explanation of the fermi paradox, except three:
1. the great filter. the great filter means that at least one of the variables required for a intergalactic civilization, - is incredibly improbable. if we're lucky, then it's one of the variables we've passed:
- maybe it's biogenesis, or multicellular, or sentience. in this case, we're safe, we've already done those things, and the galaxy is our oyster.
- on the other hand, maybe it's something we haven't encountered yet, maybe it's self annihilation in face of technological progress, maybe it's an astronomical event that wipes out all forms of life every now and then. in which case, we may be screwed.
2. we're too fucking primitive: we don't see them simply because.. our ability to do so sucks, and we're just impatient bastards running too fast for conclusions. maybe their are ways to make energy so obvious that every species figures out eventually, that make the very idea of visibly large dyson spheres ridicules. maybe their are so many better ways to communicate that make radio signals and our SETI program with it into a bad joke. or maybe we just haven't covered enough of the sky to find the aliens just primitive enough for us to be able to detect. maybe we even already detected them, and are clueless about interpreting it as alien life.
3. natural selection: this is a little baby concept-solution of mine, suggesting that if their are multiply alien civilizations, survival of the least visible would be the rule. simply put, speed is the biggest advantage in space, because no matter how resourceful you are and how much firepower you are capable of, if they can go the slightest bit faster, they will eventually be able to access enough resources concentrate enough fire power to counter it. encountering another civilization means you are taking the risks that they might be hostile and be able to go faster then you. in that case, any civilization that revealed itself takes a huge survival risk, and is more likely to die off. it is thus possible that the galaxy is full of civilizations that are doing their best to hide from each other. do some of them get detected? probably, maybe sometimes with peaceful results. but if they spent hundreds of millions of years working on it, it probably takes a lot more to do so then what we have to offer.