We all know Newton was the first to propose that the force that makes an apple fall is the same as the force that carries the Earth around the sun but he never mastered it, Einstein did. Gravity doesn't work just be pulling, large objects bend space time (like the earth in the picture behind) and other objects roll through it.
General relativity does have problems though. It can not deal with objects of a tiny size like atoms. For atoms there is a whole different set of laws.
The science of the small objects. Quantum Mechanics has 3 forces, the strong nuclear force small nuclear force and electromagnetism. Not everything is predictable, mostly random motion. In fact its so unpredictable because it doesn't even follow the laws of time. It also predicts that there is more than one universe. Quantum Mechanics is the math of probability and small movement.
The Four Fundamental Forces and Unification
The forces are: Gravity, Electromagnetism, Strong Nuclear Force, Weak Nuclear force. Gravity is described in general relativity are described in quantum mechanics. All four forces are proven to be true and the math for each has been proven to be true but the math of the gravity and the other three forces don't work together. This is a big scientific problem if the forces of the universe do not coincide then how can we truly further advance. This has lead many to try and discover the missing piece that finally unites all the forces.
String Theory or The Theory of Everything
String Theory states that there are tiny things, the smallest things in the universe in fact, called strings that vibrate and effect the forces and the universe drastically. String theory isn't so simple though, it comes with many radical other ideas. For instance string theory suggests that there are 11 dimensions and other dimensions and predicts the behavior of black holes. String theory has promise mathematically to be the one theory of everything but it has one huge problem. The strings suggested to exist are so small that it would be impossible to scientifically test them to prove their existence. Still String Theory has enough mathematical potential to be taken very seriously and maybe someday will be proven to be the Theory of Everything