string theory
What if the fundamental things in nature are not tiny points but tiny loops and threads of vibrating energy, so small they look like points to us? Pluck such a string in different ways and it resonates in different modes, and each mode of vibration appears to us as a different particle, an electron here, a photon there, a graviton in yet another. That is the bold central image of string theory: one kind of object, playing every note in the particle spectrum, offering a single framework for all of physics.
String theory replaces the zero-dimensional point particles of quantum field theory with one-dimensional extended objects, strings, whose length is around the Planck scale, about 10^-35 metres. The vibrational modes of a string correspond to particles with definite mass and spin, and remarkably, one mode is always a massless spin-2 particle with exactly the properties of the graviton, so string theory automatically includes quantum gravity. Mathematical consistency (freedom from anomalies) forces the theory to include supersymmetry and to live in ten spacetime dimensions; the six extra dimensions are imagined to be curled up, or compactified, too small to see.
String theory's great appeal is that it is a candidate for a unified, finite quantum theory of gravity, replacing the point-like interactions that make gravity non-renormalizable with smooth, spread-out string interactions. Its great difficulty is testability: the natural energy scale is the Planck scale, utterly out of experimental reach, and the way the extra dimensions curl up is not unique, giving an enormous landscape of possible universes with no known principle to select ours. It is beautiful, influential mathematics whose status as physics remains, honestly, unconfirmed.
A guitar string plays many notes depending on how it vibrates; a fundamental string does the same, but each note is a particle. The lowest, gentlest mode of the string is massless and spin-2, and it behaves exactly like the graviton, which is why gravity emerges from string theory rather than having to be added by hand.
One string, many vibrations, many particles, gravity included.
String theory is a leading candidate for quantum gravity, not an experimentally confirmed theory. Its natural scale is the Planck energy, far beyond any accelerator, and the vast landscape of ways to compactify the extra dimensions means it does not yet make sharp, unique predictions we can test.