I think the reason no-one knows is that it is an ill-posed question: you're inserting an assumption that there's a thing called "space" which must behave in one of these two ways.
In fact, the phrase "space expands" is just a metaphor for the mathematics, which describe a set of coordinates expanding, so that points within the coordinate frame move apart as time goes by.

By way of analogy: suppose I take a "mind-expanding" drug. During my mind-expanding experience, does extra mind appear, or does my existing mind simply get bigger? Most people would feel that such a question makes no sense, since they recognize that in this context both "mind" and "expanding" are metaphors for something we don't properly understand. I think the analogy with "space expanding" is quite tight, apart from the fact that we us a mathematical theory to describe "space", and subjective knowledge to describe "mind".

Grant Hutchison