Music is (un)filled with silence.
On the macro level this silence occurs between movements, sections, passages, phrases, and even single notes: gaps in sound-time. On the micro level it occurs within every sonic event, at the infinitesimal points when the sound waves attain zero amplitude (zero crossings).
Silence can be pure (absolute) or coloured (relative). Pure musical silence occurs when all sound generation halts: a fortissimo climax followed by a two-second pause. Coloured musical silence occurs when the sound level drops to a whisper, sometimes to the very threshold of inaudibility: a dropout where a loud passage gives sudden way to a very soft sub-bass drone.
After spending time in an anechoic chamber, Cage declared that there was no such thing as absolute silence: “I heard two sounds, one high and one low. When I described them to the engineer in charge, he informed me that the high one was my nervous system in operation, the low one my blood in circulation.”
Silence in music is normally thought of as absence. One has a block of sound and removes chunks from it, a form of subtractive synthesis. This is the sculptor’s way of composing; you chip away bits of the piece until it finds its ideal form.
To create music using the silence-as-absence approach, you begin by writing a passage (groove, melodic line, chord progression) of substantial density. You then listen, over and over, to your passage until you find where it wants to pause, hold back, stutter, breathe. Then you silence audio by reducing its volume to (near) inaudibility, or insert silences between events.
You can also regard silence as a presence in music. This is similar to additive synthesis, in that sound accrues upon silence. It is the painterly way of composing: you start with a blank canvas (silence) and add colours and shapes to it.
To create music using the silence-as-presence approach, you begin with emptiness and add sonic objects to it: single notes, phrases, loops, layers, sections, etc. In general, pieces composed this way tend to be sparser and softer than those composed using the silence as absence approach, since the starting point is pure silence. If you truly love silence, you don’t want to impose unnecessary sound on it.