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
It’s not something that can easily be ELI5’d, because even the experts only have theories. Dark matter or “anti-matter”, mass that has the opposite effect of gravity (when things pull together), is still largely unexplored, because we have never witnessed it, only witnessed the effects of it.
Think, if you will, of theorising the existence of a bear in the woods, purely by seeing its tracks and its poop, but never being able to see the bear itself.
This is the way we know about Dark Matter.
It seems to have the opposite properties of regular mass. In this way, think of the two ends of a magnet. We know a lot about the end that pulls, that’s where we get our knowledge of gravity from, but there’s this whole opposing force that pushes, that we’ve barely touched upon in research.
I’m by no means an expert, but I’m fascinated by all this stuff, so I’ve read/watched a lot about it. I’ve always got more to learn though (as do we all!) so I’m very passionate in talking about it!
Space is not defined as the absence of matter, space is simply – well – the space in which all the galaxies, star systems etc. exist.
While this mass density of space is fairly homogeneous, that is: space has roughly the same amount of matter in in everywhere, there are some small variations. Some areas of space have more galaxies in it than others.
This is the variation of density the article is referring to.
Space has ~2 particles per cubic meter, it’s not a total absence of things. I think the easiest way to understand would be to think about nebulae, which are only visible because a large group of stars has the particles attracted into one spot and the light makes it visible. There’s a lot of stuff in space that we just can’t see because it’s dark, but in a nebula? It practically glows. TL;DR, space isn’t empty it just has a lot of dark stuff and gravity can concentrate that dark stuff.
>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.
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