The Fermi Paradox is alive and well.
As our sciences mature, and as the search for extraterrestrial intelligence continues to fail, the Great Silence becomes louder than ever. The seemingly empty cosmos is screaming out to us that something is askew.
Our isolation in the Universe has in no small way shaped and defined the human condition. It is such an indelible part of our reality that it is often taken for granted or rationalized to extremes.
To deal with the cognitive dissonance created by the Great Silence, we have resorted to good old fashioned human arrogance, anthropocentrism, and worse, an inter-galactic inferiority complex. We make excuses and rationalizations like, ‘we are the first,’ ‘we are all alone,’ or, ‘why would any advanced civilization want to bother with us backward humans?’
Under closer scrutiny, however, these excuses don’t hold. Our sciences are steadily maturing and we are discovering more and more that our isolation in the cosmos and the dearth of observable artificial phenomenon is in direct violation of our expectations, and by consequence, our own anticipated future as a space-faring species.
Indeed, one of the greatest philosophical and scientific challenges that currently confronts humanity is the unsolved question of the existence of extraterrestrial intelligences (ETI's).
We have yet to see any evidence for their existence. It does not appear that ETI’s have come through our solar system; we see no signs of their activities in space; we have yet to receive any kind of communication from them.
Adding to the Great Silence is the realization that they should have been here by now -- the problem known as the Fermi Paradox.
The Fermi Paradox
The Fermi Paradox is the contradictory and counter-intuitive observation that we have yet to see any evidence for the existence of ETI’s. The size and age of the Universe suggests that many technologically advanced ETI’s ought to exist. However, this hypothesis seems inconsistent with the lack of observational evidence to support it.
Largely ignored in 1950 when physicist Enrico Fermi famously asked, “Where is everybody,” and virtually dismissed at the seminal SETI conference in 1971, the conundrum was given new momentum by Michael Hart in 1975 (which is why it is sometimes referred to as the Fermi-Hart Paradox).
Today, 35 years after it was reinvigorated by Hart, it is a hotly contested and relevant topic -- a trend that will undoubtedly continue as our sciences, technologies and future visions develop.
Back with a vengeance
A number of inter-disciplinal breakthroughs and insights have contributed to the Fermi Paradox gaining credence as an unsolved scientific problem. Here are some reasons why:
Improved quantification and conceptualization of our cosmological environment
The scale of our cosmological environment is coming into focus. Our Universe contains about 10^11 to 10^12 galaxies, giving rise to a total of 10^22 to 10^24 stars. And this is what exists right now; there have been a billion trillion stars in our past Universe. 
The Milky Way itself, which is considered a giant as far as galaxies go, contains as many as 400 billion stars and has a diameter of 100,000 light years.
Improved understanding of planet formation, composition and the presence of habitable zones
The Universe formed 13.7 billion years ago. The Milky Way Galaxy formed a mere 200 million years later, making our Galaxy nearly as old as the Universe itself. Work by Charles Lineweaver has shown that planets also began forming a very long time ago; he places estimates of Earth-like planets forming 9 billion years ago (Gyr).
According to Lineweaver, the median age of planets in the Galaxy is 6.4+/0.7 Gyr which is significantly more than the Earth’s age. An average terrestrial planet in the Galaxy is 1.6 Gyr older than the Earth. It is estimated that three quarters of earth-like planets in the Galactic habitable zone are older than the Earth.
We have a growing conception of where habitation could be sustained in the Galaxy. The requirements are a host star that formed between 4 to 8 Gyr ago, enough heavy elements to form terrestrial planets, sufficient time for biological evolution, an environment free of sterilization events (namely super novae), and an annular region between 7 and 9 kiloparsecs from the galactic center that widens with time. 
The discovery of extrasolar planets
Over 240 extrasolar planets have been discovered as of May 1, 2007. Most of these are so-called “hot Jupiters,” but the possibility that their satellites could be habitable cannot be ruled out. Many of these systems have stable circumstellar habitable zones.
Somewhat shockingly, the first Earth-like planet was discovered earlier this year orbiting the red star Gilese 581; it is 20 light years away, 1.5 times the diameter of Earth, is suspected to have water and an atmosphere, and its temperature fluctuates between 0 and 40 degrees Celsius.
Confirmation of the rapid origination of life on Earth
The Earth formed 4.6 Gyr ago and rocks began to appear 3.9 Gyr ago. Life emerged quickly thereafter 3 Gyr ago. Some estimates show that life emerged in as little as 600 million years after the formation of rocks.
Growing legitimacy of panspermia theories
There is a very good chance that we inhabit a highly compromised and fertile Galaxy in which ‘life seeds’ are strewn about. The Earth itself has been a potentially infectious agent for nearly 3 billion years.
Evidence has emerged that some grains of material in our solar system came from beyond our solar system. Recent experiments show that microorganisms can survive dormancy for long periods of time and under space conditions. We also now know that rocks can travel from Mars to Earth.
Discovery of extremophiles
Simple life is much more resilient to environmental stress than previously imagined. Biological diversity is probably much larger than conventionally assumed.
Developing conception of a biophilic Universe in which the cosmological parameters for the existence of life appear finely tuned
As scientists delve deeper and deeper into the unsolved mysteries of the Universe, they are discovering that a number of cosmological parameters are excruciatingly specific. So specific, in fact, that any minor alteration to key parameters would throw the entire Universe off kilter and result in a system completely unfriendly to life. The parameters of the Universe that are in place are so specific as to almost suggest that spawning life is in fact what the Universe is supposed to do. 
Cosmological uniformitarianism implies that that anthropic observation need not be and cannot be specific to human observers, but rather to any observer in general; in other words, the Universe can support the presence of any kind of observer, whether they be here on Earth or on the other side of the cosmos.
Confirmation of the early potential for intelligent life
My own calculations have shown that intelligence could have first emerged in the Universe as long as 4.5 Gyr ago -- a finding that is consistent with other estimates, including those of Lineweaver and David Grinspoon.
Refinement of evolutionary biology, computer science and systems theories
Evolution shows progressive trends towards increasing complexity and in the direction of increasing fitness. There has also been the growing acceptance of Neo-Darwinism.
Advances in computer science have reshaped our conception of what is possible from an informational and digital perspective. There is the growing acceptance of systems theories which take emergent properties and complexity into account. Game theory and the rise of rational intelligence add another level to this dynamic mix.
Development of sociobiological observations as they pertain to the rapid evolution of intelligent life and the apparent radical potential for advanced intelligence
And then there is the theoretic potential for a technological Singularity, digital minds, artificial superintelligence, molecular nanotechnology, and other radical possibilities. There is also emerging speculation about the feasibility of interstellar travel, colonization and communication.
In other words….
There are more stars in the Universe than we can possibly fathom. Any conception of ‘rare,’ ‘not enough time’ or ‘far away’ has to be set against the inability of human psychology to grasp such vast cosmological scales and quantities. The Universe and the Milky Way are extremely old, our galaxy has been able to produce rocky planets for quite some time now, and our earth is a relative new-comer to the galaxy.
The composition of our solar system and the Earth itself may not be as rare as some astronomers and astrobiologists believe. These discoveries are a serious blow to the Rare Earth Hypothesis – the idea that the genesis, development and proliferation of life is an extremely special event. It’s also a blow to Brandon Carter’s anthropic argument which takes a very human-centric approach to understanding cosmology, suggesting that our existence as observers imposes the sort of Universe that only we can observe.
Finally, the Universe appears capable of spawning radically advanced intelligence – the kind of advanced intelligence that transhumanists speculate about, namely post-Singularity, post-biological machine minds. Given intelligent life's ability to overcome scarcity, and its tendency to colonize new habitats, it seems likely that any advanced civilization would seek out new resources and colonize first their star system, and then surrounding star systems. Indeed, estimates place the time to colonize the Galaxy anywhere from one million to 100 million years.
The fact that our Galaxy appears unperturbed is hard to explain. We should be living in a Galaxy that is saturated with intelligence and highly organized. Thus, it may be assumed that intelligent life is rare, or, given our seemingly biophilic Universe, our assumptions about the general behaviour of intelligent civilizations are flawed.
A paradox is a paradox for a reason: it means there’s something wrong in our thinking.
So, where is everybody?
 Hart, M. H. "An Explanation for the Absence of Extraterrestrial Life on Earth," Quarterly Journal of the Royal Astronomical Society, 16, 128-135 (1975).
 This list, which is not intended to be a complete re-affirmation of the Fermi Paradox, was inspired and partly adapted from: Ćirković , Milan M. and Bradbury, Robert J. "Galactic Gradients, Postbiological Evolution and the Apparent Failure of SETI", New Astronomy, vol. 11, pp. 628-639 (2006).
 "How many stars are there in the Universe?" European Space Agency, Space Scientist, February 23, 2004: http://www.esa.int/esaSC/SEM75BS1VED_index_0.html.
 Hanson, R. 1999, “Great Filter,” (preprint at http://hanson.berkeley.edu/greatfilter.html).
 See Harvey Mudd and S. E. Levine: “Mass of the Milky Way and Dwarf Spheroidal Stream Membership.”
 Gonzalez, G., Brownlee, D., and Ward, P. 2001, “The Galactic Habitable Zone: Galactic Chemical Evolution,” Icarus 152, 185-200; Lineweaver, Charles H., Fenner , Yeshe, and Gibson, Brad K. 2004, “The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way.”; M. Noble , Z. E. Musielak , and M. Cuntz: 2002, "Orbital Stability of Terrestrial Planets inside the Habitable Zones of Extrasolar Planetary Systems"
 "A Rush of New Planets," Astrobiology Magazine: Jun 02, 2007: http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=2351
 "All Wet? Astronomers Claim Discovery of Earth-like Planet," Scientific American, April 24, 2007: http://www.sciam.com/article.cfm?articleID=25A261F0-E7F2-99DF-313249A4883E6A86&chanID=sa007
 See Stephen J. Mojzsis: http://spot.colorado.edu/~mojzsis/
 Raulin-Cerceau, F., Maurel, M.-C., and Schneider, J. 1998, “From panspermia to bioastronomy, the evolution of the hypothesis of universal life,” Orig. Life Evol. Biosph. 28, 597; "Encore: Great Debates Part VI," Astrobiology Magazine, Aug 19, 2002: http://www.astrobio.net/news/article254.html
 The Wikipedia entry on the Fine Tuning argument has some good links and references: http://en.wikipedia.org/wiki/Fine-tuned_universe
 Dvorsky, George: 2006, “When Did Intelligent Life First Emerge in the Universe?” http://sentientdevelopments.blogspot.com/2006/06/when-did-intelligence-first-emerge-in.html;
 Ward, P. D. and Brownlee, D. 2000, Rare Earth: Why Complex Life Is Uncommon in the Universe (Springer,
 Ćirković ,