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I've been reading Sagan's Cosmos recently and wondering about this question. Actually, there are some hypotheses:

1. Civilisations developing technology (radiostelescopes) also invent ways of destroying themselves (nucelar bombs). They destroy themselves so often, that the number of civilisations co-existing within our galaxy at a given time is about 10. (from Sagan's book)

2. Civilistations concentrate on listening instead of broadcasting due to limited resources/funds. Nobody talks, everybody just listen. (a Polish book)

3. Civilisations capable of detecting Earthlings are very advanced. They are aware of our existence but they do not want to interfere and prefer to observe. (Sagan's book).

4. We are living at the border of a region in space that has been recently cleansed of all life by a violent supernova. Eartlings were too far but other life in the vincinity was destroyed (my humble assumption).

5. There are colonisers gradually proceeding from one system to another but they have not reached us yet - life is common but intelligent life is quite scarce (Sagan's book).

What do YOU think? Which option is most likely and why? Or maybe some other option not mentioned here?

Regards,

Karol P.