String theory is a theoretical concept in physics where scientists assume that reality is made of infinitely vibrating strings.
These strings are much smaller than electrons and atoms.
Some scientists believe that gravity is created by the vibration of these strings. They also explain why gravity is stronger near massive objects like Earth or Sun.
This also explains why the force of gravity is weaker between two masses separated by large distances.
There are several versions of string theory, each with its assumptions and predictions.
For example, some versions predict that there are an infinite number of extra dimensions, while others suggest that ten dimensions exist.