In general, solutions to Fermi's paradox come down to:
Life is difficult to start and to evolve to an intelligent and technologically advanced stage and we're the only one in the galaxy.
Advanced civilizations destroy themselves on short timescales.
As the revised Drake equation solution (N<<1) shows, it seems that hopefuly, the first solution is the answer.
Here is a list of the various assumptions along with notes pointing to related problems and contradictions.
They are here unobserved
They were here and they left material evidence:
UFO's, alien artifacts, ancient astronauts. Problem: evidence for aliens is non-existent.
The aliens are here and they are keeping us in a well designed zoo (cut off from all contact) or there is an treaty preventing contact with young races. Problem: this scenario can't be tested. Only one ET could break the embargo.
They are us
Humans are the descendents of previous alien civilizations. Problem: where are the original aliens? where are all the other alien civilizations?
They exist but...
They didn't have enough time to reach us
The speed of light slows communications and makes space travel too long. Extraterrestrial's messages may not have reached us yet. Problem: the galaxy exist for billions of years, even if one ET civilization formed a few million years before us, the galaxy would be already filled with Bracewell-Von Neumann probes.
They are sending signals, but we do not know how to listen.
The galaxy is filled with killer robots looking for signals. ET is keeping low. Problem: no evidence of berserkers coming after us.
They have no desire to communicate
ET has no interest in communicating with lesser intelligences. Problem: with millions of possible civilizations, some of the extraterrestrial ones would be interested in our species:.
Civilizations only have a limited lifetime, they are all dead.
Dangerous particle physics
Nanobots -> the gray goo problem
They developed different mathematics
Mathematics is the universal language, but humans may have a unique system of mathematics that ET cannot understand. Problem: then where are those incomprehensible signals?
They do not exist because
We are the first in the galaxy
Life is new to the galaxy and evolution takes time. We are the first intelligent civilization. Problem: the Sun being an average star, if other stars formed a million years ahead of us, then they would be a million years ahead of us technologically speaking.
Planets with the right conditions are rare
Habitable zones, proper orbit for liquid water are rare
The galaxy is a dangerous place (asteroid impacts, gamma-ray bursters, etc)
The Earth/Moon system is unique (large tides needed for molecular evolution)
Life is rare
Life's genesis is rare
Intelligence and tool-making is rare
Language is unique mankind
Technology and science is not inevitable