Maybe I’m misinformed about this. But I was always told matter cannot be destroyed or created. But I have been told space is constantly expanding. Is this expansion of space not made up of “matter”? Where does it come from?
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At the cosmic scale, energy conservation doesn’t hold true. This is exactly because of (or related to) the expansion of the universe.
Dark energy makes up 68% of all matter/energy in the universe and it’s quantity is directly related to the size of space. So, as the universe expands there is more dark energy in the universe and hence more total energy.
Another way to reason about it is with Noether’s theorem, which states that every conserved quantity has a corresponding symmetry. For energy, this is time symmetry. With the expansion of the universe we no longer have time symmetry at the cosmic scale, which means we also no longer have energy conservation.
I should note that dark energy is a frontier of modern physics. That is to say that it’s not well understood. So this explanation is based on current understanding, which is known to be incomplete.
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