The Drake Equation proposes to predict the number of technological civilizations whose radio emissions are detectable. It is the basis of SETI.

But the universe is silent.

The great filter is proposed as a possible reason, some great mechanism that filters out most possible civilizations before they get the chance. A variation of the They all destroy themselves theory.

Or there is the von Neumann probe theory: that we wouldn't know it if we saw it, and we wouldn't know where to look theory. Not eactly why the universe is silent, but in the same general line of thinking.

Many years ago, reading a science fiction author, I think Stanislaw Lem, I came across a much simpler theory, called the butterfly hypothesis. Simply put, it states that the stage of radio technology, the period in which a civilization actually wishes to communicate with another is relatively short. And the period of desire to communicate not longer.

Very quickly, the civilization enters the butterfly stage, and flys away.

I suppose this aligns with the notion of the singularity, though I had never heard of it until I started here on Everything. And I know that I hadn't read about it when I read about the butterfly hypothesis. It could certainly be one reason for a civilization turning inward.

We are still in the birth of a radio civilization. We have no idea what things will be like, even in 15 short years (if that is the date of the expected singularity).

A purely pedantic response to Rook:

By silent, I mean devoid of intelligent communication, either directly to us, or that we can overhear