We are all aware that the universe is a very big, diverse place.  Our own galaxy, the Milky Way, contains somewhere in the range of 200 to 400 billion stars.  Estimates of the number of galaxies in the entire universe go up to about 500 billion, each containing it's own hundreds of billions of stars.  These are staggering numbers, and the odds of us being the only life-producing planet in the galaxy, much less the universe, are infinitesimally small, pretty much to the point of being impossible.  Certainly, for life as we know it, a planet has to be a certain distance from its sun, and many other elements have to be in place for life to exist and expand, not something that happens around every sun or even every 50, 100, or even 1,000 suns.   But it undoubtedly does happen, even if only occasionally.  Accepting this as fact, it would follow that some life-producing planets would still be at the very early stages of life, some would be in the middle, and some would be advanced, with only the advanced having the capability to venture out or at least send beacons to worlds outside their own.  So even if life is teeming in our galaxy and beyond, the worlds with beings advanced enough to make contact (and close enough to attract our attention) are probably fairly limited, though still, the odds are high that there are at least a few.   So where are they, and why have we come up with no hard-core,  indisputable proof that extraterrestrial life exists?  Below are some of the prevailing theories:

Advanced civilizations are self-destructive 
This is a chilling theory whereby once a civilization advances to a certain point, it inadvertently (or advertently) annihilates itself.  This is not so far out, as it involves stuff we read about in the papers every day here on planet earth, including overpopulation, scarcity of resources, communicable diseases, nuclear bombs, biological terrorism, and air or water pollution, to name a few.  There are virtually unlimited ways to either mess each other up or mess up our environment to the point of no return as we become more technologically advanced, and we may be at a make or break point even now on planet earth.  If a an alien civilization has indeed gotten to the point of being able venture out to find life in other places, it may be that its efforts just didn't last long enough to accomplish anything meaningful due to its own inclination to self-destruct.  Due to the distances involved in sending and receiving messages over the vastness of space, a civilization would have to watch, listen, and transmit for a very long time to increase chances of being detected by a world elsewhere in the galaxy or beyond.  Of course, our inclination to self-destruct would have to be a trait shared by most advanced life in the universe for this theory to be valid, and we have no evidence that this is the case so far. 

Periodic eradication of life by the planet itself
This one is not so far-fetched.  We already know that our planet is a dangerous one, with super  volcanoes, ice ages, mega-quakes, solar flares, and meteor hits that occur on a periodic basis, eradicating large amounts of life in the process.  We can only assume that any other planet with an environment conducive to advanced life forms would also have these types of scenarios.  It takes a living, breathing planet to evolve living, breathing life forms.  Therefore, life could be a "one step forward, two steps back" type of thing, where beings evolve to a certain point only to be devastated by a natural disaster of global magnitude, returning to a non-technological state for at least a while.  Our technology has only been in it's currently recognizable state for around the last 150 years, and at any point a global catastrophe could occur that would set us back to being farmers or even cave-dwellers.   Again, technology would have to be sustained for a long period of time in order to detect or be detected by another civilization.

Alien beings are out there, but they choose to remain silent
Here on earth, we have programs that are actively looking for evidence of life on other planets, mainly by scanning the skies for intelligent-sounding radio waves and looking with telescopes for telltale signs of habitable places.  We haven't made a huge effort to transmit messages out into the universe for others to pick up, however, we have made a few notable attempts, including the Voyager gold records, which many scientists think were a particularly unwise move.  The problem is, we don't know who is going to intercept any messages we send.  They could be received by a highly advanced, benevolent civilization that will help bring us into a golden age, or they could be received by a malevolent civilization with the technology and the will to destroy us completely.  It would take a worldwide consensus (not likely) to formally embark on any kind of focused messaging into the universe because the consequences (good or bad) would be shared by the entire planet.  There are just too many unknowns at this point, and the prudent thing is to just passively listen and look for clues rather than aggressively send out signals.  It would seem reasonable that other civilizations might have the same concerns, and also passively watch and listen.  The result is that everybody is listening but nobody is sending, a real possibility.  There are, however, inadvertent signals we send out with radio and television and the like that could be picked up by someone looking in the right place at the right time.    They might be watching Star Trek reruns on planet Zircon for all we know, but that's unlikely, since we have only been transmitting radio signals for about the last 100 years.  In cosmic terms, 100 years is the blink of an eye, and it's very possible that nobody has detected us thus far, at least via radio waves or television.   Even if  they have detected us, it's not unreasonable to think that they might very well stay cautiously silent at least for a while to assess what we are all about before announcing their presence.

The government is withholding information about extraterrestrial life
This is pretty much a given.  The government withholds information about everything.   The question is to what degree are they withholding information.  If you look back through history, there is some fairly compelling evidence for the presence of extraterrestrials all the way back to the cave dwellers, who drew pictures on cave walls that look just like our modern alien depictions.   Ancient civilizations built amazing structures that we would be hard-pressed to duplicate today, and they supposedly did it with only the most rudimentary tools.  There are places like the Nazca lines in Peru that don't look like anything until you see them from the air.  And of course, there are the modern stories - jet pilots who are followed by something that can outmaneuver anything we have, astronauts who see strange things while in space, and even Presidents who themselves have experienced weird unexplained phenomenon.  Then there is Project Blue Book - the government's attempt to explain UFO sightings, 20% of which were never explained satisfactorily.  And of course, there is Roswell and Area 51.  When the crash occurred at Roswell, the government came right out and said it was an alien crash, only to backpedal the next day and say it was a weather balloon.  These little pieces of evidence here, there, and everywhere do tend to make one wonder what they really know.  The evidence strongly suggests that they are aware that something is going on beyond the ordinary, but it also suggests that they don't deem it a huge threat.  We are not presently ramping up our military or our space programs to combat a threat from the great beyond, and although we have experienced remarkable advancement in the last 150 years or so, we pretty much know that the seeds were planted by the hard work of the Einsteins of the world, not by alien intervention.  So the bottom line is that the government almost certainly does know a lot more than they are telling us about extraterrestrials, but whatever it is they know, they apparently don't deem it to be an imminent threat, at least for the time being.

Extraterrestrials are too alien
We are constantly working towards unlocking the secrets of the universe, but in the grand scheme of creation, we are just babes in the woods despite all our gee whiz technological advances.  The truth is, we don't know what it's like to visit another galaxy or even another star in our own galaxy.  We have a rudimentary grasp of the physics of the universe, but most of our knowledge is derived from theory and not practical experience.  We tend to think of extraterrestrials as being somewhat like us, but what if they aren't?  They might live by a completely different set or rules than we do, living in higher or lower dimension and vibrating on a different frequency entirely.   It could be that they are all around us and we simply don't recognize them for what they are or even see them at all.  They could be passing through a wormhole right in the middle of our living room with us completely unaware of them and them of us.  It could be that they are so fast that we can't see them or so slow that we can't see them.  Maybe every solar system has it's own unique brand of "life."  If they don't generate radio waves and we are looking for radio waves, we are simply barking up the wrong tree, and it could be that they are signaling with some technology we haven't even discovered yet. 

Conclusion: 
It is worth noting (as far as the general public is concerned) that so far there is no concrete evidence that we have been visited by extraterrestrials in our immediate vicinity (the parts of the solar system we have explored).  Governments might know more, but if they do, they are staying remarkably silent about it. We do have some anecdotal evidence by various credible people, but this does not constitute proof.  Space is such a big big place that its breadth and scope are almost beyond comprehension.  It takes a radio wave 8 minutes to reach the sun, 4 years to reach even the nearest star, and 120,000 years to go across the galaxy.  Not to mention close to a million years to reach even the closest galaxy outside our own.  So distance alone could very well be the reason we are not aware of any extraterrestrial civilizations, or it could be a combination of factors above, such as distance and the length of time advanced civilizations exist.  If someone on the other side of the galaxy started sending out radio waves 50,000 years ago, they wouldn't reach us for another 70,000 years, and then we would have to be listening in the right place at the right time. We wouldn't have a clue as to whether they still existed or not after receiving such a message.  Although receipt of the message would constitute proof of extraterrestrials, it probably wouldn't help all that much as far as getting to know how they lived or what they thought (depending on the contents of the message, of course).  This would be similar to finding a message in a bottle from 120 years ago.  You would know that someone existed that created the message, but that individual would be long gone and the only information about them would be the contents of the message itself.  It is worth noting that at this time, science is making great strides in pinpointing where extraterrestrial life may reside.  A planet was discovered in late 2011 that is in the right position related to its star and has a very good chance of harboring life.  This is about 20 light years from us.  No word so far as to whether any intelligent-sounding signals have emanated from it, but it is an exciting find nonetheless.  As we learn more about what constitutes "outer space", we are realizing that space itself contains all the ingredients for life.  Indeed, we are all "star children," having been born of the very same stuff out of which the universe itself was created.  This strongly suggests that we are not unique in the universe.  At some point, if we hold together long enough as a civilization, we may very well find a way to bend space-time, reducing or eliminating the distance problems, but that is the stuff of science fiction as of this writing.   So in conclusion, at least for the time being, all we can really do is keep looking and hope that we can find that needle in the haystack that we know has to exist  just beyond our reach.   

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