The Drake Equation was developed by Frank Drake in 1961 as a way to focus on the factors which determine how many intelligent, communicating civilizations there are in our galaxy. Try the Drake Equation!
http://www.activemind.com/Mysterious/Topics/SETI/drake_equation.html
http://en.wikipedia.org/wiki/Drake_equation
The Fermi Paradox: The belief that the universe contains many technologically advanced civilizations, combined with our lack of observational evidence to support that view, is inconsistent. Either this assumption is incorrect (and technologically advanced intelligent life is much rarer than we believe), our current observations are incomplete (and we simply have not detected them yet), or our search methodologies are flawed (we are not searching for the correct indicators).
http://en.wikipedia.org/wiki/Fermi_paradox
Very simply, Drake says there should be lots of aliens but Fermi says OK how come we have heard no sign of them via radio / light waves?
I think it was Asimov who speculated in one of his essays, that the reason why we have not heard any aliens via electromagnetic radiation (or whatever medium) is that technologically advanced aliens would use focused electromagnetic radiation (or whatever medium) and wide-band unfocused electromagnetic radiation (or whatever medium) only takes place for a short time in the technological evolution of a species' communication.
However even if this argument is true, it does not provide the reason why technologically advanced aliens would not want to display some sort of welcome beacon (as pulsars were for a bit once speculated to possibility be).
If there are technologically advanced intelligent life-forms, it's dubious they would not have the resources to make broadcasting devices, but there certainly could be physically and intellectually advanced intelligent life-forms that do not have broadcasting devices.
Another possibility is that most if not all technologically advanced intelligent life-forms self-destruct before long and hence don't put out much electromagnetic radiation for extended periods.
Another possibility is that most if not all sufficiently advanced intelligent life-forms soon metamorphosize into the equivalent of non-corporal life (what some might call godlike) and are indifferent to the physical world as we know it.
Such a life form might be hard to detect by us and might have abilities that would seem to us to be godlike or magical.
I am not suggesting that such life forms exist, but personally I cannot see why they could not exist, and I would argue that non-corporal life is much more likely than a non-corporal supreme being such as defined in the Abrahamic religions.
Conscious thought in humans is a function of electrochemical signals between neurons in the brain, so why couldn't a structure that performs a similar function be maintained by energy signals (electrical and/or photonic etc.) in an energy matrix. I am not suggesting I have any direct knowledge of non-corporal intelligence, it's speculation, but it's fun.
A more likely shorter term consideration would be artificial intelligence, which one might argue, is to some degree non-corporal, because it's really the software not the hardware that is the intelligence. But that to some fair degree is more a question of semantics and perspective.
Also just to keep things straight I do mean "non-corporal" in the sense that a Science Fiction writer might use it to describe an intelligence without conventional physical form.