On a purely pedantic note, I would like to point out that the universe itself is not silent--in fact, thanks to all the very small things whacking into other things out there in the blackness, the universe makes a great deal of "background" noise that radio astronomers are quite familiar with.

Another possibility not discussed here is that we are alone in the universe. It's not a popular theory, and personally I don't believe it, but logical rigor demands that we consider it.