I believe I understand that space is constantly expanding “outward” in all directions and this is likely due to an event such as the Big Bang. Can someone explain what space is expanding *into* and how exactly the “space between stuff is also expanding” in the sense that A and B are getting further apart but the space between A and B is also expanding.
Is that simply a function of two objects distancing themselves therefore the space must be getting larger, or is the space actually expanding in between two objects regardless of either object being on either side?
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I think it is best understood as it being a fundamental property of space that every bit of space is constantly creating a little bit more space. That is, if you have two stationary objects 1 metre apart with no forces at all in between them, that metre of space will very slowly create new space which will make those objects drift apart.
For human-sized amounts of space, the effect is trivial enough to be unnoticeable, and is completely dominated by other more significant forces — even the force of gravity, which is itself usually unnoticeable for human-sized objects completely overcomes the effect of expanding space. But the amount of space created is proportional to the amount of space already present — 10 metres of space creates 10x as much “new space” per second as 1 metre of space does resulting in 10x as much “intrinsic drift”. 100 metres creates 100x as much, &c.
But in our universe, we have things like distant galaxies (i.e. not even the closer-to-us galaxies) that have ridiculous amounts of space between them. For those galaxies there is so much space between us and them, every metre of which is constantly churning out a tiny little bit of new space, but there are so many of those metres that the total effect becomes more significant than even the gravity between galaxies. The intrinsic drift from expanding space is what is mostly responsible for their motion relative to us, with any other motion being just a small correction on that.
It is a little bit off to say that this motion was “due to an event like the big bang”, which gives the impression of some explosion in the past pushing everything away and that distant galaxies are still “riding” the push from that past event. It is more like the big bang was caused by this intrinsic property of space. In earlier times and higher energy densities the “space making more space” effect was stronger and the drift was faster.
Note that this doesn’t assume that “all of space” was at some particular size at the time of the big bang. It could still have had infinite size. It’s just that space, of whatever size, was hotter and more full of stuff than it is now.
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