With all the people posting here, I'm surprised that no one has yet to mention Fermi's Paradox.
NOTE: Fermi's Paradox covers the possibility of
intelligent life existing in the
Milky Way Galaxy.
Fermi's Paradox Link
This website contains everything required to know about Fermi's Paradox, including some arguments (or rather solutions) against it. But for those who are too lazy to click on a link and read it, I'll try to sum it up in a nutshell.
The Formula
Through some inventive thinking and a few complex calculations, Fermi came up with a paradox confronting those who believe that someone is out there listening to us. Fermi realized this:
1) Given the theoretical age of the universe (let's just assume ~15 billion years) AND
2) Given the theoretical age of the Earth (~4.6 billion years) AND
3) Given the probability in the "Drake Equation" combined with the probability of intelligent life developing into an affluent civilization AND
4) Given the fact that it takes, using sub-light propulsion (the least effective from of space travel -- or, in other words, in the worst possible case), 50 million years to conquer the galaxy, STARTING from one end of the galaxy to the other,
Fermi concluded that there should be
200 million advanced civilizations that should exist if they were all to form at the same time. Or, between 50 thousand and one million advanced civilizations currently existence in the Milky Way today. Even if these civilizations didn't all co-exist at once, the number is still staggering. According to the given probabilities of the Drake equation and the given probability of advanced civilizations forming, every 70 years an advanced civilization is born.
So that means...
Earth SHOULD have been colonized by now. The probability of life, and of intelligent life is near certain in our galaxy if we use our current knowledge of the Milky Way. At the rate of civilizations popping up, it seems highly unlikely that there not be a single race out there more intelligent than us.
Now you might say, well, it takes an incredibly long time to conquer the galaxy from end to end. Heck, that's 50 million years! However, 50 million years is like a few weeks comapred to how long the Earth has been around. Even if we eliminate the baby stages of Earth's life, living organisms still appeared about 3.8 billion years ago. Life on land appeared 400 million years ago. That's quite some time.
And that's assuming a race is using primitive technology. After a few thousand years have passed, alien technology could theoretically far surpass ours. Fusion technology, hydrogen propulsion, antimatter combustion -- whatever. It sounds preposterous to us, but then other races have been around far longer than we have. In the last decade, our technology has nearly doubled in advancements. Give it a few thousand more years, and you may find yourself zipping through the Solar System at expotential speeds.
Taking all of this into consideration, why haven't we heard anything yet? Where are these so called aliens who dorm with us in the Milky Way? Even if we haven't seen them yet, why hasn't something like the Hubble detected anything? A blip here? A radio signal there? Alas, nothing. Dead silence.
Okay. Well, what about...
.....The fact that sustaining life on a spaceship is difficult!
For humans. We have no idea what the life expectancy of an alien is. The Giant Tortoise has lived into an age of 177 years. If humanoid tortoises existed, you can bet dollars to doughnuts that they'd live a long time.
But then there's the problem of oxygen, food stores, and general boredom. Okay, scratch manned travel. How about AI?
von Neumann Colonization
Essentially, von Neumann probes are self-replicating probes that are born out from the mother vessel (by using raw materials found in space), continually multiplying. Imagine a bacteria replicating. First it splits from one to two, then two to four, four to eight, and so on. All the while, these von Neumann probes would retain their originally programmed AI and set out on their courses. Even if a few of them happen to die out, these probes would construct themselves at such a rate that common logic would dictate that we should've seen a few by now. Or at least detected their presence. Well, we haven't.
.....Maybe the aliens are observing us from afar!
This isn't Star Trek, so you could drop all of that right now. Even if there were an expansionist civilization out there that was peaceful, the odds are there are also a few that are aggressive and imperialistic in nature. And why wouldn't they be? Our planet could look mighty tasty to the right race. Humans could be used for slave labor. Our planet could be a stepping stone to another planet. Even if there WAS an alien race attempting to protect us yet not allow us to have knowledge of them, it wouldn't take much to detect them. A simple ET sighting and the cover is blown. Might be a little hard to stay undercover, especially when at war.
.....What if the civilizations destroyed themselves?
There are all sorts of reasons that species could destroy themselves. Heck, were looking at one right now. Pollution, Galactic war, Asteroids, Gammy Ray Bursts, Black Holes, Supernovas, Lack of resources, e.t.c. However, is this very likely for EVERY form of life in our galaxy to die out like this? Wouldn't there be remains? Some sort of residue? Even one race that could launch ONE von Neumann probe could easily be discovered, even after they've been wiped out. Why haven't we found such a thing?
.....We are the aliens!
A possibility, but there would have to be some sort of evidence of such a thing. Genetic tampering, artifcats, left over debris from colonizing vessels, e.t.c. Someone would have to cover their tracks VERY well to accomplish such a task. And let's put all the "Mission to Mars" and "Prime Directive" theories to sleep. With all the civilizations that there SHOULD be, for some reason we haven't detected one iota of their existence.
Conclusion: All assumptions of things like widespread galactic disaster, or cloaking spaceships aside, the final conclusion is this: We are alone in this galaxy as far as intelligent life is concerned. Otherwise, where is it? Other galaxies? Possibly. But intergalactic travel is pretty far fetched. So you can keep your fingers crossed about a UFO landing in your backyard, but I'm sure as **** not holding my breath.