[skeptical]

A few years back, I carried out a simple exercise. I was interested in the question of how long it would take humanity to totally colonise the Milky Way galaxy to the point of overpopulation.

The answer depends on the assumptions you put into the model. I assumed no FTL travel, but a cruising speed between stars of 0.2C. I discovered that population growth was not limiting. The times involved were so great that population size would grow more than enough. I also assumed that, if a star system had no habitable planets, humans would simply build artificial habitats in space, such as giant rotating cylinders, using local raw materials. Thus, my assumption that expansion was not limited by planets available.

The answer, depending on assumptions, was somewhere between 400,000 years and 10 million years.

The shortest time (400,000 years) was based the assumption that humanity went bananas and devoted itself to little more than expansion. This would be achieved by sending billions of probes at 0.2C to all corners of the galaxy. Each probe would contain frozen embryos and advanced robots able to create new habitats, and thaw, and educate the embryos.

Anyway, back to aliens. This gives us a time scale for alien expansion, also. If we postulate just one alien species in our galaxy that is aggressively expansionist, then it would colonise the entire kaboodle within 10 million years. This time is a mere eye blink against the age of the galaxy.

Hence the Fermi Paradox. Where the hell are they?

Simplest answer is that advanced life forms are sufficiently rare that no such aggressively expansionist species has evolved in our galaxy.