What is it made of? Is it physical or just a concept? Why does mass bend it?
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I don’t see this in the other answers, so I will provide some extra context on spacetime. The concept of spacetime had been around previously, but it was formalized with Einstein theory of General Relativity. In essence Einstein said that the two are inexorably related. Einstein realized that massive object bend space and therefore objects traveling through that space. The gravity of massive objects will also bend light. Because the speed of light is constant, it must mean that time has to be bent as well, and equally so.
There have since been proof that extremely massive objects do bend light as the light travels around it, we call that gravitational lensing.
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