I have no doubt there are other intelligent species somewhere out there. The universe is so damn massive its incomprehensible. So just the sheer random odds that a few planets somewhere have life-supporting features, and that life randomly started would tell me (when considering the size of our universe) that there is more life than just us.
However, this is a double-edged sword. The sheer size of the universe and how spread out it is, makes it just about impossible for us to ever make contact with any other life forms. The best odds of another intelligent, developed society comes from other nearby galaxies, which have a lot of suns relatively the same size as ours. But these galaxies are so far that they are quite un-reachable. It takes light itself 42,000 years to travel to the nearest galaxy, which is actually a very small galaxy, thus the odds are smaller for it to have life. But I will assume there is sentient life there, for the sake of illustration. So considering our space shuttles go ~17,000 mph, it would take ~1.7 BILLION years for us to go that distance. For reference, the universe itself is ~13.8 billion years old.
Now granted, if there were a hypothetical species out there that has developed technology taking them much faster than we ever could, even if they go 100 times faster than us, thats still 17 million years of traveling, assuming they knew in advance exactly where they were going and how to get there. This includes finding us, somehow determining we are intelligent, plotting a course, and creating some kind of sustaining, self-contained ship to hold an entire race of people to breed, grow food, etc etc, all with one common goal to reach us. And when they get here, after all that travelling, I bet you a brand new 'busa that they would want to sit down, have a beer, and watch the ****in game, before they even begin to think about conquering/demolishing us...
*edit*
and this self-sustaining ship would have to grow and create from scratch the fuel they need, on the fly, in order to do any kind of maneuvering over those millions of years. You arent carrying that much fuel with you. Plus, you would need extremely good calculations to pinpoint exactly where our planet will be 17 million years from the time you left, taking into account that as you observe the planet from your home planet, you are seeing where it was 42,000 years ago, not where it is currently.
Believe me, I love hearing about ufo sightings, and these conspiracy theories, because they are very entertaining. Very much like a sci-fi novel, or a movie, but they are nothing more than that.
