Space-time is not expanding. In General Relativistic models, space-time exists as a static 4-manifold. To an observer,
space appears to be expanding, but that is an illusion created by the "motion" of the observor along her world-line -- that is, as time passes.
Consider this analogy: Space time is a 2-manifold, that is, the surface of a sphere. Like all of us, she can't help but experience time passing -- this is represented by her motion south along a line of longitude. Looking sideways is her experience of "space". So at a given point in time, she has moved down in latitude, and the latitude represents that time on her watch. A line of constant latitude is the
spatial universe that she sees; that is, to her the universe is a circle.
Now, a little bit later, she is a little further south; now the circle of constant latitude is a large circle. To the observer, the universe is bigger; space has "expanded". But that is because her universe at any given time is a cross-section of a higher dimensional manifold, and at different times she is seeing different cross-sections.
Spare a thought for the stay-at-home voter;
His empty eyes gaze at strange beauty shows
And a parade of the gray suited grafters:
A choice of cancer or polio. -- The Rolling Stones
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