It’s not easy being a “Theory of everything.” A TOE has the very tough job of fitting gravity into the quantum laws of nature in such a way that, on large scales, gravity looks like curves in the fabric of space-time, as Albert Einstein described in his general theory of relativity.
String theory is often described as the leading candidate for the theory of everything in our universe.
The theory famously posits that gravitons, as well as electrons, photons and everything else, are not point-particles but rather imperceptibly tiny ribbons of energy, or “Strings,” that vibrate in different ways.
Interest in string theory soared in the mid-1980s, when physicists realized that it gave mathematically consistent descriptions of quantized gravity.
M-theory looks like each of the string theories in different physical contexts but does not itself have limits on its regime of validity – a major requirement for the theory of everything.
Some macroscopic signatures of the theory that might have been seen, such as cosmic strings and supersymmetry, have not shown up.
One philosopher has even argued that string theory’s status as the only known consistent theory counts as evidence that the theory is correct.