It is a theory with no evidence.
Nope. You're (as you usually do) confusing a "theory" with some sort of
physically observable entity. You simply refuse to acknowledge what "evidence" is in its simplest form. This is why you're unable to comprehend the theory of gravity. Scientific theories are commonly defined by the phenomena they produce—such as an apple falling to the ground due to gravitational forces. An apple always falls to the ground anywhere on the planet, and has for millions of years. This phenomena is infinitely
replicable, and because of this proves that the
current scientific definition of gravity is correct. The physical phenomenon is all that's necessary to prove the theory of gravity.
Presumably, you happily accept the theories of magnetism, electricity and radioactivity? Why then reject gravity? Or string theory?
In simple layman's terms, string theory says that all the particles in the universe are really little "strings", rather than as the conventional "ball-shapes" they were normally thought of. Unlike the balls (or more accurately, points) which are
zero-dimensional, strings are
one-dimensional.
The four forces string theory conflates are the electromagnetic force, the strong and weak nuclear force and gravity. String theory is known for uniting both the physical laws of the large (Einstein's general relativity—stars, planets, and people) and the small (quantum mechanics—subatomic particles), which were previously incompatible.
Short for super sting theory, current
M-theory is an extension of string theory, in which 11 dimensions of spacetime are identified as 7 higher-dimensions plus the four dimensions in Euclidean space.
But this is not the place to talk in depth about string theory. You'll just have to accept that thousands of scientists the world over accept its premises, and are working on elaborating its agency.
—Whereas round earthers are still citing Samuel Rowbotham's fairy stories LOL.