According to Wikipedia:

"Plasma with a number density of less than one hydrogen atom per cubic metre and a temperature of millions of kelvins in the space between galaxies accounts for most of the baryonic (ordinary) matter in outer space; local concentrations have condensed into stars and galaxies." - https://en.wikipedia.org/wiki/Outer_space

I knew that intergalactic space could see densities of one atom per some human-relevant volume of space, but I had no idea that actually accounted for most of what there is!

My understanding is that there would still be quantum fluctuatons causing particles to pop in and out of existence.

Would this be protons? Can vacuum contain photons and still be a vacuum? Not sure :-)