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.
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.
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'.
>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.
I think that was exactly the point that your parent https://news.ycombinator.com/item?id=32342806 was making:
> 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.
Of course, I'm almost certainly wrong on both accounts.
> The motions of individual galaxies are hundreds of times larger than the effect of cosmological expansion