How should one rationally deal with the issue of space travelling alien civilizations? What kind of reasoning is appropriate to understand the as of today unanswered question of whether there are (other) interstellar space travelling civilizations in the Milky Way?

We have already sent probes towards the border of the Solar system. And even landed human beings on another celestial body and brought them home alive and well. If we extrapolate the 50 years of space travel, the 100 years of electronics (radio), the 400 years of physical science, to just a fraction of the biological age of humankind into the future (like a few thousands of years), interstellar travel is not out of the question for us or at least our artefacts. So I imagine two possible alternatives:

1) The Milky Way is cluttered by lots of space travelling civilizations like us and our future. Once one of them/us gets going, they'll soon be everywhere. The Sun orbits the Milky Way every 250 million years, about 2% of the age of the galaxy. Going to the nearest stars is enough to soon be everywhere. But if they are everywhere since almost always, they should be here, we should be their seed.

2) We are the only space travelling civilization in the entire galaxy, ever. But then what makes us unique? We consist of the most common elements and volatiles of the universe and our planet and star and galactic location all seem to be very typical. There's no known trace of any uniqueness here. Whatever could it be?

Are there more alternatives?

While we cannot say today which alternative is true, we should be able to at least specify the possible alternatives. But to me they all seem to be absurd! What would be a rational logical scientific approach to this apparent paradox?
That's basically the Fermi paradox. It seems likely that there are numerous civilizations in the galaxy, and yet we see no trace of them anywhere.

The Drake equation is often invoked to calculate the probability of existence of other civilizations, by compounding several other, more simple probabilities: the probability that a star has planets, that the planets are habitable, that life could arise on such a planet, etc. The proposed values so far indicate that there should be many civilizations in our galaxy.

Astronomy, just plain observations, might be one way to solve the paradox by detecting alien societies.

Broadcasting messages via radio has also been attempted, in the hope that someone might reply.

However, the only constant in this search has been the negative answer. There are some hypotheses trying to explain that:


the rare Earth hypothesis: maybe there's something very special about Earth that allows the evolution of intelligent life
the destruction hypotheses: maybe life tends to self-destruct, or maybe there are space marauders out there killing everyone
disasters: maybe things like gamma ray bursts tend to wipe out life periodically
the young Universe hypothesis: if there are very, very many universes in the Multiverse, then the vast majority of them are young. If so, in the vast majority of them there will be 0, or at most 1 civilizations. It's simply a matter of probability that we are in such a universe.
religious arguments such as the creation of Man was a special, unique act by a higher entity


Others argue that many civilizations might exist, but we don't see them. Possible reasons:


we're just too far from each other, in space, in time, or both
it's just too hard to travel between stars
we haven't been searching long enough, or we're looking or listening to the wrong things
they're not interested in us
they all go into a Technological Singularity pretty quickly and become god-like, undetectable and incomprehensible to us
they are too busy browsing their internets to be interested in the outer space (yes, that has been proposed seriously)
they are too alien for us to detect or comprehend
they are like the Na'vi in Avatar - a non-technological society
they deliberately avoid us (the Earth-as-a-zoo hypothesis)
they do try to talk to us, but evidence is being supressed by the government (yes, that is also a serious proposal)
they are here already, hiding amongst us (proposed by Iosif Shklovskii and Carl Sagan, so I guess it qualifies as serious - albeit highly hypothetical at best)


Truth is, we really don't know. All we can do now is speculate.