There in Time
Let us assume, first of all, that we are not special. That is, similar types of observer are possible in galactic or planetary states that differ from ours. Secondly, as the example of Earth illustrates, let us assume that the 20th century technological state is locally short lived. Then, even restricting ourselves to classical ideas of information transmission, we should not expect to detect alien signals until our ideas of information encoding become a lot more sophisticated.
But a truly quantum gravitational view of things would say something more profound: perhaps we haven't met alien dust because it isn't Time. In the evolution of emergent cosmic time, as viewed by Earthlike observers, the cosmos must reach a threshhold of complexity before its state can accomodate the transmission of information between galactic civilisations. Thus our state of understanding itself is the barrier that one day, if we survive, we might break.