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.