If spacetime is a fabric, what is it made of?

 Spacetime is not a physical object that is made of something. Rather, it is a mathematical concept that represents the relationship between space and time. In Einstein's theory of relativity, spacetime is a four-dimensional structure that includes both space and time, and it is used to describe the motion of objects and the effects of gravity.

The idea of spacetime as a fabric or a sheet comes from the fact that it can be thought of as a flexible, stretchable surface that can be bent and distorted by the presence of matter and energy. However, this is just an analogy and does not imply that spacetime is made of anything in the same way that a fabric is made of fibers or threads.

It's important to keep in mind that the concept of spacetime is purely a theoretical construct and is not directly observable. It is a way of understanding and describing the physical world, and it has been extremely successful in explaining a wide range of phenomena, including the orbits of planets, the behavior of light, and the properties of black holes.