ELI5: How is new space created as the universe expands? Einstein discovered that empty space is not nothing, so what is responsible for new space appearing into existence, when energy and matter cannot?

He became suspicious of it while developing his theory of general relativity. I think this answers your question:

Einstein included the cosmological constant as a term in his field equations for general relativity because he was dissatisfied that otherwise his equations did not allow, apparently, for a static universe: gravity would cause a universe that was initially at dynamic equilibrium to contract. To counteract this possibility, Einstein added the cosmological constant.

Cosmological constant history from wiki...

What Einstein thought to be his "biggest blunder" turned out to be a best fit for dark energy.

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