Von Neumann Machines
A mathematician named John Von Neumann had a theory on which this whole idea was based on. What he did is that he designed on paper machines that were capable of self-replicating thus creating new generations of themselves. These are the machines that would later come to be known as Von Neumann machines.
These kinds of machines can be built on earth following with a launch into space and then these machines would land on distant planets. After this, these machines would mine and harvest the available energy and will build replicas of themselves. They would then launch to the nearest planets and the cycle would continue.
Creation Of Von Neumann Machines
Creation of von Neumann machines demands new technologies that we humans don’t possess yet. Some of these include advance artificial intelligence, advanced propulsion system and miniaturization. If these are used to spread legit humans throughout the galaxy, we would need the ability t artificially grow biological bodies and organisms that we don’t have yet where genetic information and raw elements are used.
As a result, there will be millions of probes that would spread outward into the universe. Scholars have found that a single von Neumann machine that would be travelling at 5% of the speed of the light has the capability of replicating throughout our galaxies in the span of 4 million years and even less. Considering that our universe is 14 billion years, the time is not that long.
Intelligent Alien Civilization
If by any chance, in the last billion years, the alien civilization has created such machine then it would have reached by now but since it had not, astronomers have some theory.
- Intelligent alien civilization would not build self-replicating machines. Rather, they would just scavenge planets to their core to keep replicating.
- Absence of self-replicating machines signifies that intelligent aliens do not exist. Chances are there that they would go extinct before the idea of the development of such new technologies.
Thus, these pretty much cover the answer to the question.