Actually, I believe it to be rather impossible that intelligent extraterrestrials don't exist.
They haven't come here though, simply because there isn't a good way to do so(energy requirements and propulsion, etc. This is another discussion)
Here is my
support:
http://en.wikipedia.org/wiki/Drake_equation
Check it.
Not to mention the Universe is so vast and there are trillions among trillions of stars, thus making at least millions of planets in the universe with the same conditions as ours capable of producing life.
And this is only CARBON based life. We have no idea what other kinds of life could exist out there.
Edited:
Actually, I reread OP and it isn't really another discussion, it would fit perfectly. As far as we know it is impossible to travel faster than light(Yes I know neutrinos have, but as far as I know this hasn't be reproduced by another collider yet so we'll ignore it). The closest system(Alpha Centauri) is like 4.3 lightyears away and the planets in that system are incapable of supporting life as we know it.
Why is this important? Because from our Sun, it takes LIGHT 4.3 years to get to.
Light travels 4473 times faster than the fastest man made object ever created which is the Helios 2 space probe which reached a maximum velocity of around 150,000 mph.
It takes massive amounts of energy to accelerate anything to near light speed. The only thing we've ever gotten to near light speed are particles. Space probes and shuttles are HUGE. Now we get to my point. The energy requirements and problems creating a vessel capable of near-light speed travel are INCREDIBLE.
So not only would aliens need to have extreme methods for storing and utilizing energy for propulsion, but they would also have to wait hundreds of years to get anywhere near our planet. Not to mention, they might be an organism that couldn't survive in our atmosphere making their journey here pointless.
Of course, there are wormholes. Well, wormholes are still only a theory, and it is theorized that opening a stable wormhole requires more energy then the entire planet produces in a year, and we still don't have any idea how to keep it open.
Basically, the odds of any aliens finding us are
ASTRONOMICAL. Aside from needing extremely efficient energy storing capabilities and extraordinary propulsion systems, they would have to know where we were!
The vastness of the Universe makes it improbable for any super intelligent race to stumble upon a planet upon trillions, and almost all signals mankind has ever generated and sent out is still hundreds of years away from any reasonable destination.
If anyone can think of a counter-argument for this, PLEASE do so. It'd make my day; this is like my favorite thing to discuss.