"An additional, unexplained 43 arc-seconds/century helped demonstrate General Relativity’s correctness."

Einstein did not incorporate the motion of the Big Bang itself. Einstein left out the most important motion that matter is behaving in. He did not finish the work. He ‘just’ gave us a calculation method, correct onto itself, but not explaining why that is happening.

Einstein acknowledged that the planets are in the solar system, acknowledged that the solar system is inside the Milky Way, but he did not acknowledge that the Milky Way is itself also moving through space.

In his model, the Milky Way is therefore circling in the same space all the time. He left out the fastest speed we are involved in.

The Big Whisper model is a mechanically complete Big Bang model, incorporating a transformation event. It does explain the why.

The Lambda-CDM model desires to be a creation model, which is the reason is it mechanically incomplete.

https://fred-rick.medium.com/quarks-explain-our-universe-best-e03b7ccf72c3