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We do know why the universe is expanding. That's due to general relativity. That's well attested to high confidence.

We don't know why the expansion is accelerating. For that we have only speculation.

It’s well understood that the expansion of the universe is not “due to general relativity”. General relativity does explain some details of that expansion.

The equations of general relativity fully model the expansion of the universe, so you’d need to explain what you mean.

The expansion of the universe is largely due to the impulse provided by the Big Bang. General Relativity does attempt to explain some details regarding what’s happened after the Big Bang, but provides no insight into the Big Bang itself. Nor does it offer insight into Dark Energy, a concept that Einstein opposed.

yeah, that extra bit that Einstein was very embarrassed about having to write, the bit that says "... plus the expansion of the universe"

It explains nothing. It's just tacked on the end there.

The universe is expanding due to general relativity... in the sense that Einstein literally wrote "... plus the expansion of the universe" into his equations.

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