Back in the 17th century, Isaac Newton put forth the first comprehensive theory of gravity that worked: the law of universal gravitation. All objects with mass attracted all others with an instantaneous force determined by the distances between all pairs of objects (or particles). But when we discovered Special Relativity, and the fact that different observers would disagree about what those distance values were, we knew it couldn't be the full story.
When Einstein put forth General Relativity in 1915, it truly heralded a revolution in physics. Masses didn't just gravitate; all forms of energy did. Space and time weren't fixed and absolute, but tied together as spacetime, with properties relative to each observer. And spacetime curves and evolves based on all the matter and energy present within it. Only, when Einstein first applied it to the entire Universe, a huge problem arose. That's where the story begins.
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