2 writes
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The nature of what is expanding is not yet clear.
I think it's pretty clear by now that it's space that is expanding.
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Space has properties of energy. Then it has mass.
You are mistaking what goes on in space as oppose to what space itself is.
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We simply do not have the ability to detect it yet.
Space is space. It's simply silly to try to define it in terms of other things.
We often run into the same problem with trying to define scientific terms like entropy and such to the general public. People tend to want to define such things in terms of normal everyday things. We can't do that all the time. Space is space. Gravity is gravity. Energy is energy. Entropy is entropy. If you try to define any of those terms in everyday language, you will inevitably end up losing much of its meaning.
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I hear time and again that nothing is actually composed of energy
Where on Earth did you hear this from?
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space as an expanding thing with no substance.
Space is space. It is expanding. The way we define substance is anything that occupies space. To try to define what space is by using the word substance makes no sense.
Try to think of it this way. You have an epidermis (skin) layer covering your body. We define body hair as strands of dead protein cells that grows out of the skin. What is skin? You can define it in a lot of way. But it's simply silly to try to define what the skin is in term of body hair. No amount of body hair will ever make up the skin.
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We used to thing air was "empty"
We thought wrong. Again, we didn't discover air by defining it with normal everyday language back then. We had to invent a whole new concept of matter state, mainly gaseous state. What is gas? If you want to define what gas is in term of rock and dirt, no amount of explanation will do it justice.
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For space to exist and have no mass it cannot be physical. That does not follow. Nothing is "not physical" Everything is made of something.
Space is very physical. The fact that it is not composed of "normal matter" like we know it or that it cannot be defined using everyday normal language does not negate this fact.
Anyway, the point is you can't try to understand what space is in terms of other things just like you can't understand what the color yellow is in terms of other colors like red and black. Red is red. Yellow is yellow. Space is space. Gravity is gravity. A field is a field. A bird is a bird. A reptile is reptile. You don't try to define what a reptile is with words like "rock" and "metal" do you? Why would you want to define space with substance and energy?
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