Don't expect me to be able to explain this one, but according to Einstein the negative pressure in fact 'pushes' on 'space'. Graham gave me this explanation when discussing dark energy and I compared it to the concept of invisible boxes.

"There are two aspects to the properties of matter that contribute to the gravitational attraction; one is mass density and the other is "pressure". This results from Stress-Energy tensor component in Einstein's Field Equations. The concept, in this case, is that dark energy has a negative pressure so that as the universe expands the pressure gets greater (making it expand more)."

What you're saying here, if I get you right, is that you can imagine space as something in where we have a lot of invisible boxes containing a negative pressure, restricting and constraining those boxes in a inverse direction, and as space around them grows bigger due to the 'expansion' the inverse pressure automatically become larger? The problem here is that you will need a 'box' it seems to me, to explain how this can happen?

"Yor-on, I think you have it right. The negative pressure concept means that as the universe's volume (V) increases, the vacuum energy increases (dW = -PdV) because P is itself negative. So the vacuum energy density stays the same. However the Pressure term in Einstein's field equations will dominate, and being a negative term, results in a net increasing gravitational repulsion.

However, I don't think this explains anything but rather just says what the equations reveal. I can't think of any analogy with everyday familiar behaviour that can help. I have tried to find a clear explanation on the web but have failed to do so."

So the reasoning we have when it comes to dark energy seems directly traceable to Einsteins Cosmological constant.

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And reading I realize why it would expand more between galaxies than inside. It's not true that we don't expand inside the galaxies. We do expand inside them too. But the more Space we have the more 'pressure'. So the intergalactic wastes, as Buck Roger would have expressed it, will have a greater pressure than space inside the galaxies. And also, if there is no measurable expansion inside a galaxy it might be that the 'positive' pressure of gravitation is able to take out the 'negative pressure' repulsing 'space'

I don't think he made a 'blunder'.

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But it sure gives me a headache

We now have negative and positive pressure classically, as well as a positive and negative 'energy' non-classically, aka the Casimir effect (QM)