And to my knowledge there can't be a before time.

Well, we haven't directly observed matter appearing spontaneously in a vacuum, but we have evidence to support it does happen

My layman's understanding is that virtual particles can and do emerge from vacuum, but in ways that usually cancel out before affecting anything. Occasionally it does affect normal stuff - see the Casimir effect acting on surfaces very close together.

I personally suspect this is an explanation for dark matter and a possible origin of the universe.

If there's tiny bits of stuff and anti-stuff blinking in and out of existence, anywhere there's a big fat nothing, both halves should still exhibit gravity before blipping back out. It wouldn't show up as normal matter because it spends most of its time not existing. The vacuum really is empty... on average. It just hums with enough short-lived quantum shenanigans to have nonzero mass.

And if this follows a steep curve for distribution, then it's like blackbody radiation. A hot rock will overwhelmingly emit photon wavelengths near the peak, for any given temperature, but in theory any temperature can emit any wavelength. It just happens with vanishing rarity as you get up into the spicy photons. If vacuum will occasionally fart out a particle and antiparticle, then very occasionally it should fart out two particles and antiparticles, together. And with vanishing rarity it can theoretically fart out an arbitrary quantity of mass, alongside a negation that is presumably equal. But if that's off by a little bit - if it's allowed to be off by a little bit - then an equally arbitrary quantity of mass will remain. Even if the masses have to match exactly, they could recombine in ways that produce angular momentum and never properly rejoin. And if vacuum produces gravity, well, anything that's left will accelerate away in all directions.

On cosmic timescales it's possible that matter just kinda happens. We'd be left with the question of why the fuck that's how anything works, and where all this quantum vacuum bullshit came from. But creationist cranks would have to retreat back to the first sentence. In the beginning, there was nothing. And it was slightly heavy.