system
Given the following post, respond as if you're commenting on Reddit:
human
If the universe is essentially a non-uniform vacuum, could the differences between these areas exert a force between them?

Sorry if this is a dumb or basic question. I'm not very smart so I just want to be able to wrap my head around this. Blame Mythbusters for lifting a car with a household vacuum if you must. It all made me think of dark energy and how the vacuum of space actually works.

Would a complete vacuum in one region of space exert any influence over a non-complete vacuum right next to it? Would there be pressure differences that would equal out over time as one pulled matter towards it from the other? Do the variable parts of the vacuum of space have any interrelationship at all or is it all just passive without an energy source powering one part or another as there would be with a home vacuum cleaner creating the vacuum?
gpt
I think it's easier to understand this question if you flip it around. Instead of thinking of things in terms of vacuums (with some vacuums being more vacuumy than others), it's best to think of space as being like an *atmosphere* - it's full of gas, it's just a very very thin gas. A "vacuum" can be thought of as just a very very low pressure gas.

And, as you suggest, this gas is not distributed evenly throughout the universe. The gas in the disc of the Milky Way is denser (and cooler) than the gas in the "halo" around the Milky Way, which is denser than the gas in the vast expanses between galaxies.

And this gas interacts with stuff. It collapses to form stars. These stars heat up the gas and increase its pressure. The gravity of all the stars and dark matter stops the gas from escaping away from the galaxy. The combination of pressure and gravity means you get a bit near-spherical blob of hot gas around the Milky Way - this is the gas "halo". Some of this gas is spinning, and the combination of gravity, angular momentum, and pressure, gives you a disc of gas too.

And yes, this gas does flow around, with high pressure gas trying to expand into low pressure areas. This is basically the same processes that happen within our atmosphere, just on much larger scales. But, just like our atmosphere, it's not like everything just evens itself out and then stays still forever. It gets stirred up by stellar winds and radiation and supernovae and gravity, and by the rotation of the galaxy, and all sorts of stuff. So you end up with a big turbulent mess of all sorts of stuff going on at once - which again, is pretty similar to what happens with Earth's atmosphere. Essentially, what we're talking about here is the galaxy's "weather".