The only natural, obvious, simple answer to the Fermi paradox is the one no one really wants to consider: Starfaring civilizations are just very rare - and/or they last only for a very short time. (very rare = civilizations/galaxy <<< 1, very short = lifetime civilization <<< lifetime galaxy)

This interpretation is the only one consistent with all our observations and needs no "god-of-the-gaps"-like reasoning why we don't see any aliens. There is not a single observation that implies that civilizations must be plentifull in the universe...

This solves also the following problems/observations:

- absence of any artifical signals from interstellar space
- absence of any signs of former colonization of the Earth or the solar system
- absence of any megastructures
- Late occurence of civilization on Earth (compared to the lifetime of the Earth's biosphere): typical observation if average time for evolution to develop a civilization >>> typical lifetime of a biosphere
- Typical observer argument ("Doomsday argument"): if we are typical observers*, it is unprobable that the number of people being born after us >>> people being born before us
- Typical observer argument aplied to interstellar colonies: if we are typical observers*, and we are living on the original homeplanet of our species with the galaxy still untouched by our own colonization efforts, then it is unprobable that people living in colonies >>> people living on homeplanets (otherwise the typical observer would live in a colony)

(*which is probable, as most observers are by definition typical observers...)

This view may be deemed "pessimistic", but it is not: it is just based on realism and observation. If this weren't such an emotional topic, we would have adopted this rational explanation long time ago. The belief in many "contactable" civilizations has a quasi-religious ring to it: if we don't find any evidence for it, lets find some insightfull "excuses" why they are out there but we still can't see them...

By the way, I am not suggesting to shut down SETI or the like, although I guess it will never be successfull. SETI is an experiment to test the predictions made from observations: but as far the null-result it has delivered only supports the view I have expressed here.