The Fullness of Emptiness
When we manage to empty a place in the world, when I manage to leave not an iota of matter or light, we still have something: space and time.
That space and that time allow the existence of multiple non-material entities, known as fields.
One field for each fundamental known particle.
These fields are not static; they fluctuate constantly.
So that is what the vacuum is made of: space, time, and a multitude of fluctuating fields.
True "nothingness" is dynamic.