How do we know it's not? You might say that I know that the table in front of me is solid. I can see it, I can feel it, I can rest things on it and I can try but fail to walk through it. But nowadays, I think a physicist with the right tools would be able to show us that, in fact, it is almost completely empty space.

So f***ing what? What does solidity have to do with amount of empty space? If according to your definition of solid, ice is less solid than water because it contains more empty space, your definition of solid is broken.