String theory, a theoretical framework in physics, suggests that the universe may have more than the four dimensions (three spatial dimensions and one time dimension) that we commonly experience. The specific number of dimensions in string theory depends on the formulation and version of the theory.
The most well-known version of string theory, known as "superstring theory," proposes that the universe consists of ten spacetime dimensions: nine spatial dimensions and one time dimension. This is often referred to as the 10-dimensional spacetime. In this framework, particles and forces are described by tiny, vibrating strings of energy.
Additionally, an extended version of superstring theory called "M-theory" suggests that there are eleven dimensions: ten spacetime dimensions and an additional dimension that is compactified (curled up) into a tiny, unobservable size. M-theory is believed to encompass various formulations of string theory and provide a unified description of the fundamental nature of reality.
It's important to note that the extra dimensions beyond our usual four are considered to be compactified at scales much smaller than we can currently detect or observe. These additional dimensions are hypothesized to be curled up and "hidden" within the fabric of spacetime at the Planck scale, which is roughly 10^-35 meters. As a result, their effects are not apparent in our everyday macroscopic experience.
It's worth mentioning that string theory is a highly complex and mathematically challenging area of study, and it is still an active field of research. The ultimate validity and complete understanding of string theory, including the nature and number of dimensions, is yet to be determined.