Isn’t the universe mostly vacuum?
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I want to point out that the idea of what vacuum is had changed over history, and possibly will continue to do so until physics is finally solved. Vacuum – as an imprecise idea of “nothingness” – is a fine concept in normal discussion, but when you need to get philosophical about it, such as in the above statement, you need to quantify it much more precisely, and that’s when the definition matter.
So while the universe is full of vacuum right now, what is called “vacuum” is very different to people in different era, or even the same people now with different understanding of physics. For example, the widely-believed Unruh effect will tell you that even the idea of vacuum itself is relative: an accelerating observer will see a bunch of particles in supposedly empty space.
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