How can pockets of space have different densities?

Question

I was reading this article about the theory of galactic bubbles (excuse my simplistic terminology)
[https://www.vice.com/en_in/article/4agbjn/we-actually-live-inside-a-huge-bubble-in-space-physicist-proposes](https://www.vice.com/en_in/article/4agbjn/we-actually-live-inside-a-huge-bubble-in-space-physicist-proposes)

I feel like I get it if I expel my understanding of space as a vacuum, but I was hoping someone here could explain to me how different regions of space could have different densities if, to my understanding, space is an absence of matter (totally ready to hear this understanding is wrong).

In: Physics

Answer 1

>to my understanding, space is an absence of matter

Consider that the atmosphere of the earth is a “location in space” with a lot of matter, that is way more dense than the space outside of it. And the surface of the earth is a “location in space” with even more matter that’s denser than the atmosphere outside of it.

Likewise, our galaxy is an area of space with more stuff in it than the void outside the galaxy.

We generally define space as that area outside our atmosphere, but even that is fuzzy. The atmosphere never really has a sharp boundary. It just gets thinner and thinner as you go outward. Most of what we call space is REALLY thin. Like hardly any particles per cubic meter, but not absolutely empty.

And that matter that is out there is in greater or lesser concentrations, which themselves can be held together by gravity, like our planet/atmosphere, or a galaxy, or a giant molecular cloud. These areas of space are thus more dense. Really the only reason why the atmosphere of earth isn’t “part of space” is because we define it that way. We’re certainly *in space* otherwise, and would thus represent a particularly densely packed region of it.

Now mind you, if you looked at space *on average* it is very empty. A crowded location like the atmosphere of the earth, or a molecular cloud, or a galaxy, or the corona of the sun, represents a small fraction of the total volume. Most of space is very empty, there are huge voids between objects where particles are few and far between compared to anything we know on earth. And so space when considered as a whole is a very empty place.