The curvature of space above a gravitating mass is caused by the presence of mass and energy according to Einstein's theory of general relativity. In this theory, gravity is not viewed as a force acting between objects, but rather as a result of the curvature of spacetime caused by mass and energy.
According to general relativity, mass and energy in the universe create a gravitational field that curves the fabric of spacetime. The presence of mass warps the geometry of spacetime, causing objects to move along curved paths. The more massive an object, the greater the curvature it creates in spacetime.
The curvature of spacetime is often visualized using the analogy of a rubber sheet. Imagine a heavy object placed on a stretched rubber sheet. The weight of the object causes the sheet to curve and deform around it. Similarly, a massive object curves spacetime, and other objects moving through this curved spacetime follow paths dictated by this curvature, giving the appearance of gravitational attraction.
In the vicinity of a gravitating mass, spacetime is curved such that the paths of objects, including light, are affected. This curvature is what we perceive as gravity. The curvature of space tells matter and light how to move, while the curvature of time affects the flow of time itself, causing time dilation.
To summarize, the presence of mass and energy in a region of spacetime causes the curvature of both space and time, leading to the phenomenon of gravity as described by Einstein's general theory of relativity.