If the universe is expanding outwards, are these voids also moving similarly? I just cant ever seem to reconcile the expansion theory with the theory of galaxies/voids merging or colliding.
From the perspective of any galaxy, most other galaxies are moving away from it, "outward". But some galaxies are close enough to form clusters that are gravitationally bound, which can cause them to orbit each other or merge.
We can say the visible universe is expanding outwards, because we see light from more and more distant things as the universe ages. But the actual (not just visible) universe may be infinitely large, we don't know what's beyond the range we can see (limited by the speed of light, unless we discover something radically new about physics we can never see farther). So if you look at the universe from a holistic point of view, not just from our perspective which makes us the center of the universe, expansion may not really be the best way to think about it. It's more like the size of the empty parts is increasing relative to the size of the not-empty parts.
We don’t necessarily see more light as the universe ages, nor do we see farther away. The universe is expanding, and the farther something is, the faster it is moving away from us. It can even be moving faster than the speed of light relative to us, because the space between us is growing, and more empty space between us, the more this extra stacks up. When this happens, something is beyond the visible universe - that light can never reach us.
So there are two ways we cannot see something - if the universe is not old enough for light to reach us yet, or if there is too much new universe being created in between us and the light for the light to catch up to us.
In a far enough future, there will be no other galaxies in the sky as all the matter will be too far away for light to reach us.
No really, by our current definition of 'void', does a void move or is its position and shape solely defined by the objects moving into and around it? Separately, is the void inside the universe the same as the void outside the universe, or is it fundamentally different somehow?
I don't expect we'll ever answer the second question, but the first is able to be answered by how we choose to define the term 'void'.
Voids are a description of an area that has relatively less matter density than the average of the universe, not objects themselves, so I'm not sure what you mean by a void "moving".
>Separately, is the void inside the universe the same as the void outside the universe, or is it fundamentally different somehow?
By definition they are different. Voids, as I said, are areas within the universe that have relatively less matter density. Key thing being that they are part of the universe, whereas 'outside the universe' is, well, not part of this universe.
> Voids are a description of an area that has relatively less matter density than the average of the universe, not objects themselves, so I'm not sure what you mean by a void "moving".
> Does a void move? No really, by our current definition of 'void', does a void move or is its position and shape solely defined by the objects moving into and around it?
That is, I think that they were not asking a factual question—"obviously voids can move, but do they?"—but rather a sort of ontological question—"does it even make sense to ask whether voids move?"—just as you are.
Some areas of space have a more matter than others e.g. galactic clusters, where the relative speed of the galaxies to each other is higher than the expansion. This video from the excellent Space Time series helps explain it: https://youtu.be/bUHZ2k9DYHY
We can say the visible universe is expanding outwards, because we see light from more and more distant things as the universe ages. But the actual (not just visible) universe may be infinitely large, we don't know what's beyond the range we can see (limited by the speed of light, unless we discover something radically new about physics we can never see farther). So if you look at the universe from a holistic point of view, not just from our perspective which makes us the center of the universe, expansion may not really be the best way to think about it. It's more like the size of the empty parts is increasing relative to the size of the not-empty parts.