The Universe was not "empty" even before the Big Bang.

The shape of the visible universe as a ball tells us that there was material or some kind of quantum field before the Big Bang. If we are looking at the material in the ball-shaped structure, we can imagine that material formed when the shockwave that came from the Big Bang impacted the quantum fields. In that very violent reaction, the Schwinger effect formed particle-antiparticle pairs.

There are two possibilities in this model. The shockwave that caused the Schwinger effect that formed material could come from outside the universe. Another version is that the shockwave that came inside the universe reached the outer shockwave, forming virtual particles that turned into real particles.

There are two versions of that model. The first model is that the antimatter-material annihilation is the thing that we call the "Big Bang". And the second model is that the spin of those particles was so high that it separated antimatter from matter. And that makes the antimaterial universe possible. But if the Schwingger effect formed a universe, that means there could be at least one other universe than ours. And that is the antimatter universe.

"Nearby, the stars and galaxies we see look very much like our own. But as we look farther away, we see the Universe as it was in the distant past: less structured, hotter, younger, and less evolved. In many ways, there are edges to how far back we can see in the Universe". (BigThink.com/Our Universe wasn’t empty, even before the Big Bang)

When that high-energy shockwave impacts something else, like superstrings or even another shockwave. That thing formed a dark and visible material. The thing that defined the particle as the visible or dark matter was simple. The energy level of the shockwave decreased, and at the moment when the shockwave had a certain energy level, it interfered with another shockwave whose origin was in the Big Bang. The shockwave's energy level at that moment determined whether the particle was visible or dark matter.

The fact is that this kind of impact can happen because of a shockwave that came inside the first shockwave. When we think that the Big Bang sent at least two shockwaves, those socks expanded like balls. The outer shockwave would act like a vacuum bomb. It sends energy inside it, and then the second shockwave gets more energy from it. The gravitational and quantum mechanical phenomena caused an effect that slowed the outer shockwave. And then the inner shockwave reached that first wave.

Another good explanation is that other universes send shockwaves where the wave that the Big Bang sent impacted. In that case, the material and energy that the Big Bang sent impacted waves that came from other universes. That could prove the multiverse theory.

https://bigthink.com/starts-with-a-bang/universe-wasnt-empty-before-big-bang/