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.