Creation ex-nihilo means creation from nothing. We pretend we know what we know. Sometimes it is helpful to challenge that with a model that flips things on its head.

Now make a model for physics. This is just an allegory. Think of as mass is nothing and what we think of as nothing is mass. Then the smallest particle is not a particle at all, but a discontinuity in a crystal. Energy running through the crystal will create these discontinuities as the energy can displace the lattice structure. All larger particle are just geometric patterns of discontinuities within the lattice. This model forces all of the properties of physics into the aether or the lattice structure, and you have a unified field theory when the math is worked out.

As the crystal seeks it's lowest energy state it forces the discontinuities toward each other. This is gravity. A discontinuity that places tension on the lattice at one end and compression at the other will attempt to align itself with other such discontinuities to get to the lowest energy. This is magnetism. With enough energy a new discontinuity pops into existence. This is energy transformed to matter.