This argument involves a great fallacy. The fallacy is assuming that a successful extraterrestrial civilization would be likely to spread throughout the galaxy once it was advanced enough to do interstellar travel. There is no reason to think that such a thing would occur. In fact, it may be more likely that any extraterrestrial civilization would either confine itself to its own solar system or only colonize a few nearby stars.
I will call this thesis the Rare Colonization Hypothesis. I may state the hypothesis as follows: the average extraterrestrial civilization capable of interstellar travel is likely to colonize no more than a small number of other solar systems.
Alien Rejecting Interstellar Colonization Plan
The Difficulty of Interstellar Travel
The main reason for adopting the Rare Colonization Hypothesis is the fact that judging from what we currently know about the laws of nature, interstellar travel is incredibly slow and difficult. According to Einstein's Special Theory of Relativity (which is currently regarded as established fact), it is quite impossible to travel faster than the speed of light. Outside of dense star clusters (where the heavy metals needed for life are not very common), the average distance between stars is about five light years. This means that even if you built the fastest space ship capable of traveling to another star, it would take you at least five years to get there.
In fact, there are astronautical and engineering reasons why it may well be impossible for any spaceship with living creatures to travel at more than 10% or 20% of the speed of light. One reason is the need to gradually accelerate a spaceship without creating too high a g-force for the spaceship crew. Another reason is the need to gradually decelerate the spaceship so that it would not zoom too fast into any solar system it was visiting. Another reason is that the more fuel you take along in the spaceship, the more energy it requires to accelerate the spaceship to a high speed. While there are proposals for starships that scoop up hydrogen between the stars, they don't result in speeds that are a high fraction of the speed of light.
There are all kinds of exotic proposals for faster-than-light travel involving wormholes and space warps, but they all assume that nature has a giant gift waiting for us to discover, a gift that will allow us to cheat on the law of nature saying nothing can travel faster than light. Since we haven't found that gift yet, we should assume that it probably isn't there. Your mother may have hidden $10,000 in bearer bonds under your mattress, but until you see them, you should assume they aren't there.
It is certainly possible that interstellar colonization could by done by using robots or suspended animation of a human crew or a multi-generational world ship. But, unless there is some surprising twist in the laws of physics that makes interstellar travel easy, any mission of interstellar colonization is going to take decades or centuries.
The Comparative Ease of Interstellar Radio Communication
Compared to interstellar travel, interstellar radio communication is very easy. Rather than spending trillions to launch a mission between the stars, a civilization could achieve radio and television communication across the stars for a millionth of the cost. Any interstellar rocket expedition would not produce any results known to a home planet until decades or centuries in the future, but by building some powerful radio telescopes a civilization might get results immediately.
The relative ease of interstellar radio communication makes it much less likely that civilizations would engage in extensive interstellar colonization. Why spend trillions on dangerous, risky missions between the stars, when you can get knowledge about other civilizations more quickly at a millionth of the cost?
Would There be Some Natural Tendency Leading Extraterrestrials to Colonize Many Stars?
Despite the considerations mentioned above, it is sometimes argued that there is some natural tendency that would lead a civilization to expand aggressively across the stars. It is sometimes said that humans have a natural tendency to spread their genes to as many distant places as they can. Some others argue that there is a natural human tendency to conquer everything that we can conquer. Therefore, it is argued, we should assume that alien civilizations should have a similar tendency.
But these imagined characteristics of humanity don't really exist to any extent sufficient to justify such assumptions about alien civilizations.
Do human being beings have any natural tendency to spread their genes to as many distant places as possible? Very few people actually show any such tendency. Many millions of young men have the opportunity in today's world to travel around the world, impregnating women around the globe, and spreading their genes all over the planet. Very few men do any such thing. For example, it is very rare to read about a man who has one daughter in Africa, another daughter in China, a son in South America, and another son in the United States. It is therefore false to imply that we have a natural tendency to spread our genes to as many distant places as we can.
What about the supposed human lust for territorial conquest? Very few humans have actually shown any interest in controlling very distant territories. Even if one looks at the most aggressive conquerors in history, one virtually always sees only people interested in conquering neighboring countries. Even Adolf Hitler seems to have shown no serious interest in conquering anything outside of Europe.
Do we see much in the way of human lust for conquering or colonizing worlds beyond our planet? Consider these facts:
Time when humans first landed on the moon: 44 years ago
Number of humans currently on the moon: 0
We actually see fairly little desire in humans for colonizing other worlds or spreading our genes to every distant place we can put them. So there is no reason to assume that extraterrestrials would have such a tendency.
Colonization Not Needed for Worlds to Explore
Here is another reason for thinking that interstellar colonization may be rare: any civilization capable of colonizing another star would have computer technology and virtual reality technology so advanced that it could create a huge number of breathtaking virtual worlds offering endless opportunities for virtual adventure and exploration. Imagine something like a video game, but with images as good as you see in the sharpest high-production movie, and with flawless computer-generated AI everywhere. It might be hard to get anyone to sign up for a long, risky interstellar expedition, when a person could satisfy all his lust for exploration and adventure and beauty in virtual worlds in his living room.
Diminishing Returns from Interstellar Colonization
When colonization has occurred in human history, it has usually been because the country launching the colonies was gaining clear benefits that made the colonization worthwhile. For example, the Spanish got a steady stream of gold, silver, and crops from their New World colonies which made the colonies worthwhile to the Spanish.
It seems, however, that there would be very little trade benefit to be got from interstellar colonization. The distances are simply too far for trade to be economically viable. Any civilization capable of colonizing other solar systems would have incredibly sophisticated molecular assembly technologies. The cost of sending materials or treasures from another solar system back to another solar system would be many times greater than the cost of synthesizing such products on the home planet.
What type of benefit would come from interstellar colonization? There would be a kind of “insurance policy” benefit that would come when a planet created one or two interstellar colonies. The benefit would be that the civilization that created the colony in another solar system would no longer have “all of its eggs in one basket,” and would no longer be in a situation where the destruction of its home planet would mean its extinction.
However, such an “insurance policy” benefit would diminish greatly when a planet established its second or third interstellar colony, and this benefit would diminish still further when it created its fifth or sixth interstellar colony. Once any planet existed in two solar systems, it would no longer have a risk of extinction, and would no longer have “all its eggs in one basket.” There wouldn't be any point in creating a fifth or sixth interstellar colony for an “insurance policy” benefit.
There would also be a diminishing “thrill return” from establishing interstellar colonies beyond the first or second colony in another solar system. It might be terribly exciting to create the first colony on another world, but the home planet would get a much smaller thrill from the creation of additional colonies. I'm reminded of what happened during the first few moon landings:
Viewer of Televised Apollo 11 Moonwalk: Oh my God! They're walking on the moon!
Viewer of Televised Apollo 17 Moonwalk: Not another stupid moonwalk! They're interrupting my regular quiz shows and soap operas!
In short, after a planet created one or two interstellar colonies, there would be a much smaller benefit from creating additional colonies.
The Implausibility of Colonizing Von Neumann Probes
Some people argue that extraterrestrial civilizations would create robotic machines that would go from one solar system to another, always creating mass copies of the original machine. Such machines might then fill up all habitable planets in the galaxy with copies of themselves.
I am tempted to call such a proposal the Dumbest Idea Ever. Colonizing self-reproducing robot expeditions would be like a cancer on the galaxy. Perhaps the greatest thing about a galaxy such as ours is its diversity, the fact that it presumably allows countless divergent civilizations and life forms and cultures to arise. The idea of gobbling up all the available planets and filling them with copies of one particular robotic expedition would be like replacing every canvas in the world's museums with copies of one painting. Any species capable of doing such a thing would presumably be wise enough not to do it.
The theory advanced here (the Rare Colonization Hypothesis) is a plausible hypothesis to help explain why we do not see extraterrestrials in our vicinity. There are other ways of explaining our current non-observation of extraterrestrials. It is quite possible that extraterrestrial colonization occurs very frequently, that there is some huge society of aliens, and that the Earth has been declared a kind of reserve area that is “hands off” for interference or colonization. Such an idea (sometimes called the Zoo Hypothesis) is fairly plausible, but it seems to require that there be some “controlling power” in our galactic vicinity (perhaps some empire or league of planets). The Rare Colonization Hypothesis is simpler and more plausible than this Zoo Hypothesis, because the Rare Colonization Hypothesis does not require any belief in some arrangement or law of conduct that is being followed by our galactic neighbors. The Rare Colonization Hypothesis simply requires us to believe that extraterrestrial civilizations don't do very often something that is very, very difficult to do: to establish a colony many trillions of miles away.