At the end it depends on the homogeneity of the Universe. Cosmological models assume that the Universe is isotropic and homogeneous at high enough scales. So that would cover your assumption: if it's homogeneous, the voids are included in the average density. What this study is challenging is precisely that homogeneity.
Obviously this is a completely buffoonish question made by someone who has extremely limited knowledge but...what on earth do we mean by "homogeneous at high scales"? Doesn't the existence of stars, galaxies, and galactic clusters with enormous voids in between clearly show the universe is not homogeneous in any meaningful sense? I know the phrase "high scale" is in there but, I mean, if you zoom out far enough, it seems like you could claim anything is homogeneous at a sufficiently high scale, although I don't know what that would mean on a universal level anyway, since you cannot zoom out of the universe.
In the early 20th century, before we knew about galaxies, we thought what we see in what we now know is our galaxy continued that way throughout the whole universe. There were some places with higher or lower stellar density but if you stepped back a ways you'd find that larger volumes had about the same density.
You might have to take into account those density differences if doing calculations about things going on in those regions, but when you were doing calculations about the whole universe treating it as homogenous would still be OK.
Then we learned about the Milky Way galaxy and that there were other galaxies. But it looked like galaxies were fairly evenly spread out. Like with stars within a galaxy, there were places in the universe where the galactic density was higher or lower but if you stepped out to get the bigger picture you'd still find that large volumes all had about the the same density.
So again you might have to take that into account when doing calculations in specific large regions of the universe, but what doing calculations about the whole thing it still appeared uniform enough that treating it as homogenous should still work.
Then we found that galaxies cluster. But step back and the clusters seemed to be distributed pretty evenly. So we still had a homogenous universe as far as calculations on the whole universe were concerned.
There were more rounds of this, finding larger things, but each time those larger things seemed to be pretty uniformly distributed so you could still assume homogeneity when trying to calculate universe-wide stuff.
But now with more recent mapping of these large scale things, it appears that there is some significant density variation where the regions of high or low density are large enough that if you try to step back to see if those regions are distributed uniformly at a large scale you run out of observable universe.
And so now it may be that we finally have to drop the homogeneity assumption when doing things the involve the whole universe, like trying to understand its expansion.
I guess this is still puzzling to me. From what I can gather, it sounds like our idea of cosmological homogeneity and isomorphism are roughly equivalent to my layman's conception of "things are evenly distributed and the universe, could it be viewed as a whole, is symmetrical."
But what I don't understand is why we would expect that to be true. Once we learned about galaxies, what would imply their even distribution on a galactic scale? The very existence of nonidentical galaxies / lack of obvious symmetry around us already suggests the existence of early perturbations that cause an uneven distribution and asymmetry, no?
I don't think there was an obvious lack of symmetry. In any random direction you look with a powerful telescope you see about the same number of galaxies, the same mix of types of galaxies, the same kind of variation in brightness.
It was only after decades of developing better and better ways to measure distances that we started getting good enough 3D galaxy position to recognize things like superclusters and filaments. It is only relatively recently that good enough 3D universe maps have been made to show the large voids that might might be distributed sufficiently non-uniformly that they must be taken into account.
Throughout most of that time the models based on a reasonably homogeneous universe worked. It was only relatively recently that they ran into problems. For example in the last 10 or so years a couple different ways to measure the Hubble constant were refined enough to reduce their error bars to the point that the error bars no longer overlapped.
Not to mention that “this isn’t how optics works!”
The optical properties of a material caused by its inhomogeneity doesn’t change if you look at it from further away!
I.e.: frosted glass will look frosted from any distance other than “extremely close”. The same applies to wavey or otherwise non-flat transparent or translucent materials.
The same ought to apply to the lumpiness of spacetime caused by galaxies — it shouldn’t matter if you “zoom out”, the bumps and troughs are still there, bending light the same way!
I thought this was obvious, it’s something that occurred to me at least a decade ago!
I can’t believe astrophysicists just hand-waved this away, when it clearly can’t be ignored.
At the end it depends on the homogeneity of the Universe. Cosmological models assume that the Universe is isotropic and homogeneous at high enough scales. So that would cover your assumption: if it's homogeneous, the voids are included in the average density. What this study is challenging is precisely that homogeneity.