What is there in the vacuum?

Fields. Many types of fields. They have extension, intensity, energy and may have momentum, spin, frequency and other properties. They may be electrical, magnetic, of matter, and so on. The gravitational is not a field, but the space curvature. Also there may be radiation in the vacuum, that is, just, electrical and magnetic fields oscilating. But there is not matter in the vacuum. Nor atoms, nor isolated particles. And, certainly, the space where they stay.