Physics beyond the Standard Model refers to theoretical developments needed to explain the deficiencies of the Standard Model, a few examples of which include gravity, neutrino oscillations, matter–antimatter asymmetry, and dark matter and dark energy. Then there is the issue of the mathematical framework of the Standard Model itself – the Standard Model does not tie in with general relativity, to the point that one or both theories break down under certain conditions, particularly in trying to explain space-time “singularities” like the Big Bang and black hole event horizons.

Theories that stretch beyond the Standard Model include various extensions of the existing model by way of supersymmetry, such as the Minimal Supersymmetric Standard Model (MSSM), or other explanations, such as string theory, M-theory and extra dimensions. As these theories tend to reproduce the entirety of current phenomena, the question of which is the best step toward a unified “Theory of Everything,” can only be determined via experimental study, which is why this is currently one of the most active areas of research in theoretical and experimental physics.

Wave–particle duality is a theory that proposes that all matter exhibits the properties of not only particles, which have mass, but also waves, which transfer energy. A central concept of quantum mechanics, this wave-particle duality addresses the inability of classical concepts of "particles" and "waves" to appropriately describe the behavior of objects at the quantum (super-small) scale. Standard interpretations of quantum mechanics explain this paradox objectively, as a fundamental property of the universe itself, while alternative interpretations explain the duality more subjectively—as a consequence of various limitations of the observer. By this reasoning, methods of describing matter in terms of wave-like or particle-like characteristics simply provides a relative viewpoint depending on the context. This treatment focuses on explaining the behavior from the perspective of the widely utilized Copenhagen Interpretation, in which wave–particle duality serves as one aspect of the concept of complementarily—the idea that one can observe phenomena in one way or in another, but not both simultaneously.

To correct for issues and unexplained phenomena, various extensions, revisions, and replacements of the Standard Model continue to be theorized and developed. String Theory is one of the Standard Model’s most popular reinventions, and many theoretical physicists think that theories of this nature are the next step toward a true Theory of Everything. String theory is a theoretical framework in which the point-like particles of traditional particle physics are replaced by one-dimensional objects called strings.

In string theory, the different types of observed elementary particles arise from the different quantum states of these strings. In addition to the types of particles postulated by the standard model of particle physics, string theory naturally incorporates gravity, and is therefore a good candidate for a unified theory of everything, a self-contained mathematical model that describes all fundamental forces and forms of matter. Beyond its hypothetical role in describing particle physics, string theory is now widely used as a theoretical tool in physics, and has shed light on many aspects of quantum field theory and quantum gravity.

There are many variations of String theory and its associated ideologies; theories of quantum gravity such as loop quantum gravity and others are thought by some to be promising candidates to the mathematical unification of quantum field theory and general relativity, and would require less drastic changes to existing theories. However, these solutions are looking more challenging to assert as recent discoveries places strict limitations on the effects of quantum gravity on the speed of light.

Among the numerous variations of string theory, M-Theory, whose mathematical existence was first proposed at a String Conference in 1995, stands out as having real potential to be a "Theory of Everything" candidate, most notably supported by physicists Brian Greene and Stephen Hawking. M-theory proposes an 11-Dimenstional universe composed of seven higher-dimensions of space-time in addition to the four common dimensions of space-time (four common dimensions being the three observable dimensions plus time). Proponents of M-Theory believe that the 11-dimensional theory unites all previously speculated 10-dimensional string theories and supersedes them. Though a full description of the theory is unknown and it has been criticized for lacking predictive power and being potentially untestable, research and experimentation continues to find mathematical constructs that join various surrounding theories.