According to Einstein's general theory of relativity, mass and energy determine the geometry of spacetime. Mass curves spacetime, and objects with mass move along the curves created by this curvature. In other words, mass causes spacetime to bend.
The precise mechanism by which mass bends spacetime is not fully understood at a fundamental level. General relativity describes the mathematical relationship between mass and the curvature of spacetime, but it does not provide a complete explanation of the underlying mechanism.
One way to visualize this concept is to imagine placing a heavy object on a stretched rubber sheet. The weight of the object causes the rubber sheet to deform or curve, and any smaller objects placed on the sheet will move along the curved paths created by this deformation. Similarly, mass in the universe curves the fabric of spacetime, and other objects with mass move along these curved paths.
It's important to note that the concept of mass bending spacetime is a mathematical representation used in general relativity to explain the behavior of gravity. It does not require a specific mechanism or cause for the bending itself. The theory provides a mathematical framework to describe and predict the behavior of gravity, but the underlying mechanism responsible for the curvature of spacetime is still an open question in physics.