When the universe is that big, and one Kardashev-II Civilization take over an entire galaxy within a million years, why don't we see a trace of alien civilizations? A solution to this Fermi paradox is the so-called Zoo Hypothesis, loosely translated the Zoo hypothesis. Do we live in a zoo, where the aliens laugh at us?
The earth as a zoo
The Zoo Hypothesis, conceived in the 1970s and popularized afterwards in the visionary science fiction series Star Trek, argues that there is a kind of 'prime directive'forbids advanced species from contacting primitive species such as ours. So there are indeed aliens and they have found us for a long time, according to this theory. The problem is that they do everything they can to avoid being found by us, as long as we are not yet ready for contact.
Motives for the 'zookeepers'
In the Zoo Hypothesis model, it is assumed that the alien species or species (a single species is more likely here) believes in cultural diversity. Much like the multiculturalism currently in vogue. The idea is that an intelligent life-form, raised entirely in cultural isolation, will develop unique solutions to social and technical problems. Who knows, they may discover completely new theories where their own kind does not come up. For example, a clear interest shared by all universe inhabitants: a way to escape the presumably extremely wretched fate of this universe in the distant future.
Pros and Cons of the Zoo Hypothesis
The Zoo Hypothesis offers a watertight explanation for the Fermi paradox and makes no assumptions about the technology of our surrounding aliens, other than that they have a high form of organization. A surrounding alien empire or federation with a high degree of stability, which would require faster than light communication or a very slow pace of life.
The disadvantage is that the Zoo Hypothesis has a number of weaknesses. Even if one alien broke the embargo, it would have had profound consequences for human history. Air vehicles and nuclear weapons to finally settle a battle between Harappa and other Vedic principalities. Or a desert prophet bluff with a flying device. Or maybe kidnap someone, place nano implants and turn them into your ventriloquist dummy, so you can create your own cool doomsday cult can begin. That idea. In short: as an alien joker you can fully indulge yourself in that dull monkey species on Terra.
The jury is still out. We may never discover the truth.