M-theory
String theory has been developed because of a very important problem that has existed for almost 100 years. Albert Einstein's theory that describes the universe on very large scales (it is called general relativity), and it disagrees with two theories that describe things on very small scales (they are called quantum mechanics and the standard model). There are also problems with the Standard Model: it includes about 20 numbers that seem to have no explanation; it has too many basic particles - some scientists think it needs to have fewer; and it does not include gravity, which is needed to explain weight.