This is a question that tampered with the brains of the most brilliant people on earth.
Still no answer.
The facts? Trillions of stars, trillions of possible planets. Billions of years the age of universe.
So there must be a lot of intelligent civilizations. And they must have had the time to communicate with us.
Then, where are they? This is the well known Fermi Paradox (that is the where are they questions since they must be not so rare according to
One possible answer to this is that they have destroyed themselves. Intelligent life may not be so rare but intelligent life that reaches global destructive power capabilities and does not destroy itself may be incredibly rare. We have witnessed some times already with the menace of a nuclear war. We are now facing the near future scenario of grey goo (self replicating nanorobots that destroy everything in their way living only their mass called grey goo).
Another scenario is that we have not searched too much. SETI has not scanned a great portion of the universe yet. We are scanning for radio waves. There may not by any radio ways because advanced civilizations do not use them. So we may be searching the wrong way.
Or maybe in the wrong place. Advance civilizations should have advance need for energy. So we should be looking towards place of high energy in space and not towards places where biochemistry might flourish. We should also think about what an advanced civilization look like. Like a fragile human being? Or like a substrate that could travel through space without any limitation.(remember pathfinder that travelled to Mars? well something like this with a brain like us 
would be a more preferable vehicle for space travel)
Or maybe like a matryoshka brain? Based on a Dyson sphere, one that could surround our sun and use it’s energy for computational intelligence, we should be looking for infrared waves where stars should be (that is how a Dyson sphere should look to us).
Then maybe the speed of light is unavoidable and advanced civilizations are not interested in traveling million of years to come to us primitives.
Who knows? The quest will be continued…
Remember the recent exoplanet that was discovered? The one that may possibly bear life?
Watch the extraordinary documentary from google video below to travel with your mind on how a robotic mission to this planet might look like…
