It's not composed of physical matter, as far as I or anyone else can tell.

[Sabriel pours the cocoa into two mugs, and hands one to Rachel.]

Calling it a spirit is probably accurate, but we don't actually know its location, or enough about its history to say if it was alive once, alive now... or if 'alive' is the wrong way to think about it.

[Sabriel approves of the calm approach. Although having been that age... she's worried about Rachel trying to hunt down the Silence.]

If there is a way to hurt it- or seal it away, it almost certainly involves magic.