Our own universe is a substrate and a stratum.
As a stratum, our universe contains nuclear particles, photons, protons, electrons, neutrons, atoms, molecules, planets, stars, galaxies, clusters of galaxies, super-clusters
of galaxies, quasars, pulsars, black holes and all other objects of observational astronomy. Astronomers have extended observational capabilities practically to the boundary of our own universe.
As a substrate, our universe is a solid sphere of densely packed particles upon which the aforementioned stratum is directly superimposed. This solid sphere fills all of the
space of our universe up to a distinct boundary.
A substrate is best understood in terms of a detailed analysis of its particles. In quantum field theory, the substrate corresponds to the hypothetical quantum vacuum.