According to the theory of general relativity, developed by Albert Einstein, spacetime is curved due to the presence of mass and energy. This theory describes gravity as the curvature of spacetime caused by mass and energy.
Einstein's theory proposes that objects with mass or energy create a gravitational field that warps the fabric of spacetime around them. The presence of mass and energy causes spacetime to curve, much like a heavy object placed on a stretched rubber sheet would cause the sheet to sag or curve around it.
The curvature of spacetime affects the motion of objects within it, including the paths of light and other particles. Objects and particles tend to follow the curvature of spacetime, moving along what are known as geodesics. The curvature of spacetime is what we perceive as gravity. In other words, the gravitational force we experience is a result of the curvature of spacetime caused by mass and energy.
This concept is often illustrated using the analogy of a ball rolling on a curved surface. The curvature of the surface affects the motion of the ball, causing it to curve or orbit around the massive object.
The idea of curved spacetime is a fundamental concept in general relativity and has been supported by various experimental observations and measurements. For example, the bending of light around massive objects, such as stars, has been observed and confirmed, providing evidence for the curvature of spacetime.