Scientists estimate we can only see roughly 4% of whats around us. We probably can't see the aliens if they are out there.
What 4% are you talking about?
If you mean the observable Universe as compared to the actual Universe, the observable Universe is probably a lot more than 4% of the size of the actual Universe and even if it isn't it's still quite big enough for lots of other civilizations to exist.
If you mean normal matter and normal energy as opposed to dark matter and dark energy, well, again it doesn't matter (no pun intended) because our civilization is made of normal matter and that's the kind we're talking about. It's not like the aliens can hide themselves behind dark matter or something.
Space colonization would be next to impossible unless faster-than-light communication can be achieved. If it takes 2.4 years to send a radio signal to the nearest star other than the Sun, how can we expect to communicate with distant colonies or ships?
Maybe we won't. Each colony and ship could hold a copy of an artificial intelligence program which governs the people and always retains the same basic goals, so everything would run smoothly even though no one would know what their interstellar neighbors were doing.
Besides, quantum mechanics may yet give us ways to communicate (and possibly even move) faster than light. Of course, in quantum terms, 'faster than light' doesn't really mean anything, because there is no motion involved; the information just is in one place and then is in another place (or alternatively is in both places at once).
You have to take into consideration that
A) Not every planet is habitable.
B) Not every star has planets.
The colony ships themselves could probably sustain life while orbiting a star. Perhaps large colony ships would be used that would stop at a planetless relay star and then send on a smaller ship to the final destination.
The thing is, look how huge the universe is. Travelling across it would take longer than the universe has been in exsitence (Dunno how to spell that word;)). And also, other extra terrestial beings would have to evolve, they can't just start off really advanced. It can be compared to a game of starcraft. It's not like you suddenly spawn 30 battlecruisers, you have to build yourself up, all the while protecting yourself from various things like Zerg in the game, meteors and diseases on the planet.
I think that if there is someone out there, they haven't had the time to reach us.
The thing actually is, the distances within galaxies are very small compared to the age of the Universe, and galaxies are still very big (our own galaxy has about 200 billion stars). As the travel time goes up linearly, the area you can cover in that time goes up by the cube, so it very quickly (in terms of the age of the Universe) becomes enormous, with the potential for many civilizations within that area.
Let's assume we are within a few thousand years (a negligible amount of time) of achieving interstellar travel technology which can allow us to colonize at 10% of the speed of light. That means we can cross the Milky Way in only a million years. In other words, if anyone else in our galaxy was even only one million years ahead of us (one million years being only 0.00007 times age of the Universe), we should already have met them.
There's a big difference between life and sentient life... Who's to say that ET organisms out there will be intelligent enough to even have a civilization?
Again, there are millions of 'they'. For every hundred or thousand or whatever that didn't develop intelligence, there'll be one that did. And one is enough.