Summary: The best hypothesis we have at this moment is the extra-terrestrial hypothesis.
Let's play around with the Greenbank equation. In our galaxy alone, there are an estimated 100 to 400 Billion stars, but let’s make the number 200 billion. Out of 200 billion, perhaps 66 billion, or one third of them have planets. Let’s say, just to be conservative, that our own solar system has about double the average, which we’ll make, of 5 planets. That makes 330 billion planets. And of those, perhaps 10% (33 billion) are planets capable of sustaining some form of life (note: If our solar system is any indication, the number should be higher than this, as life could theoretically exist in one form or another at one time or another on at least two and probably more like three of our own planets and moons. --i.e. Earth, Mars, and Europa, but we’re not counting moons here.) If we say that life will actually evolve on 1 in 100 of those habitable planets, (again, it looks as though the numbers in our own solar system may be much higher—possibly moving up from 1 in 3 to 2 in 3 or conceivably 3 in 3) we now have 330 million inhabited planets. Of those, perhaps one in a thousand will develop an intelligent form of life. We now have 330,000 planets with intelligent inhabitants. Suppose fifty percent of those will actually develop some form of advanced technology such as our own. That brings the number to 165,000. Now, if 1 in a thousand of those planets manage to find a way to live peacefully together, that will give us 165 basically immortal civilizations in any group of 200 billion stars produced by our galaxy. In other words, a technologically advanced peaceful civilization is likely to move (at least in part) off-planet, and possibly colonize other worlds, and other star systems, making them effectively immortal. This means that their technological progression will continue indefinitely, depending on whether or not there are limits to what the civilization can do or wants to do with technology. If there is a possibility that the speed of light problem can be circumvented, as can be theorized even by us evolutionary primitives (e.g. wormholes, gravitational bending of space, quantum theories of nonlocality, etc.), then eventually, a civilization who wants to figure out how to do it, will. And once they do, they can go anywhere instantaneously.
Now, remember that we are currently talking about a group of 165 hypothetical immortal civilizations produced by the 200 billion currently visible stars in our little galaxy. We must remember that there have probably been stars before these and after these, that may have produced more civilizations than this. But lets say that these two hundred billion stars are the only ones we have to work with in our galaxy. We have 165 immortal civilizations, more than likely with hundreds of thousands if not millions or billions of years of advanced technological experience under their belts. Keep in mind that our own civilization has had less than 30 or 40,000 days worth of experience with "advanced technology."
So where are these guys? This is known as Fermi’s Paradox. As Fermi pointed out, even a civilization as technologically primitive as our own, given the resources and desire to do so, could conceivably colonize the entire Milky Way Galaxy in about 50 million years, assuming technology didn’t advance during that time making the process faster. The universe is thought to be around 15 billion years old. That’s enough time for somebody to make such a snail-paced journey 300 times. Certainly, if there are more than a few civilizations like our own out there, at least one of them would be here now. And as we have seen, it is not unlikely there are a couple of hundred of them out there who are ahead of us by millenia piled on millenia. Where are they? Most SETI astronomers have an answer like this: “We know they're not here, but it would be foolish for us not to gamble on them being out there somewhere when the odds seem to be so good that they are, and with such an incalculable payoff if we do make Contact.” Don’t get me wrong. I’m a big supporter of SETI’s efforts. I agree completely with the main thrust of their answer, because I may be wrong about UFOs. Maybe Fermi’s paradox really is a paradox, but I don’t think we can rush to that conclusion.
UFOs are real. That much is certain. Objects of a currently unexplainable nature have been seen in the sky for thousands of years. They have been recorded in pre-modern times by priests, military commanders, possibly even by pre-historic artists in caves around the world. In modern times they have been repeatedly photographed, tracked on radar, sighted, chased, and even fired upon. Planes and pilots have been lost in pursuing them (see military quotes page). They are, as General Nathan F. Twining put it, “something real and not visionary or fictitious.”
The only question is what these things in the sky are. Certainly there are hoaxes. Certainly a majority of sightings are misidentified natural or man-made phenomena. But hoaxes do not shoot down pursuing aircraft. Natural and man-made phenomena do not (as far as we know) hover over nuclear missile bases in the United States and the Soviet Union and turn missiles on and off. The best hypothesis we have at this moment is the extra-terrestrial hypothesis. Those guys from the equation… I think they’re here. This needs to be taken seriously. The SETI guys are right in their answer to the Fermi Paradox, but I am too, and for the same reason.