How can the space between galaxies be expanding?

First we must accept that Universe is 4 dimensional (the simple model for the purpose of discussion here) and 'time' being one.

It (Universe) is like a balloon that is being blown (filled with air) and is expanding. Assuming it to be spherical expansion implies its radiaus is increasing with time. Hence radius is the 'time' axis. Normal Earthly balloon is 3 dimensional and the surface is 2 dimensional. For the balloon of Universe (4 D) the surface is actually a hyper surface of 3 dimensions, the physical world that we are all familiar with. If we mark dots (with ink) on the surface, and call them 'galaxies', we find that the distance between the dots increasing when the balloon expands. I think this is the model that is appropriate.

It also explains the location of the center of Universe at any point of time. This question gets asked many times in Y!A.

The answer to your question is not really known, and here's why:

In classical General Relativity, space has no substance. There are "spacetime events", points in spacetime where something particular is happening at a certain place and time in spacetime. And the distance between spacetime events, in time and in space, can be defined and determined. So the spacetime events exist in a geometry, and that geometry is governed by the dynamics of the Einstein Field Equations. In the context of General Relativity, we can state that the separation between galaxies is getting larger with time, and we can say that this means "space is expanding", but that statement is vacuous (pardon the pun). There's nothing that's getting bigger.

Now, in Quantum Field Theory, space is not "nothing". Space is filled with fields, and those fields have some energy even in the lowest energy state. So making more space requires making more fields. But that's about as far as we've gotten---things get very fuzzy in terms of theoretical explanations in "Quantum Gravity", and there is no prediction or understanding of "Dark Energy", which seems to be a part of space. Quantitatively, there's no way to connect the expectations of quantum fields with the amount of "Dark Energy", although string theorists have been trying hard for more than a decade. So you see, your question is at the forefront of current research in physics.

The expansion is actually accelerating. It seems that space itself has a tendency to expand. It's got something to do with the negative pressure of vacuum energy, quantum fluctuations, virtual particles, and such. The energy that drives the expansion of the universe is also called Dark Energy.

Because as the universe expands the three large spacial dimensions that we encounter every day are also expanding. Space is not infinite according to some theories, it apparently has an edge and as time goes on from one perspective the space dimension is still expanding while others believe it is shrinking. But in your question if you believe it is expanding it is because of the dimensions.

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