Based on the information given, the central idea is that Newton had important theories about gravity, but Einstein developed a new theory of space and gravity.
It should be noted that a central idea simply means the main idea that's portayed in a story. It's simply what the reader wants the author to know.
Newton had important theories about gravity, but Einstein developed a new theory of space and gravity. He thought that space was like a blanket that bends and twists. After that, scientists began to theorize that black holes could exist
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