In fact, there is more structure below the level of the nucleus. Protons and neutrons are not truly "elementary" in nature but are actually made of subparticles called quarks. (The name derives from a meaningless word coined by novelist James Joyce in his book Finnegans Wake.) According to current theory, there are precisely six distinct types of quark in the universe, paired with six distinct types of lepton—the electron and two related "electron-like" particles (called muons and tails,) and three types of neutrino. The most massive, and most elusive, of them—the so-called top quark—was discovered at the Fermi National Laboratory (Fermilab) in Illinois in April 1994. The strong nuclear force is itself just a manifestation of an even more basic force that binds quarks to one another.

On the face of it, one might not imagine that there could be any deep underlying connection between forces as dissimilar as those just described, yet there is growing evidence that they may really be just different aspects of a single basic phenomenon. In the 1960s, theoretical physicists succeeded in explaining the electromagnetic and weak forces in terms of the electroweak force. Shortly thereafter, the first attempts were made at combining the strong and electroweak forces into a single all-encompassing "superforce."

Theories that combine the strong and electroweak forces into one are generically known as Grand Unified Theories, or, less formally, GUTs. (Note that the term is plural—no one GUT has yet been proven to be "the" correct description of nature.) One general prediction of GUTs is that the three nongravitational forces are indistinguishable from one another only at enormously high energies, corresponding to temperatures in excess of 10²⁸ K. Below that temperature the superforce splits into two, displaying its separate strong and electroweak aspects. In particle-physics parlance, we say that there is a "symmetry" between the strong and the electroweak forces that becomes broken at temperatures below 10²⁸ K, allowing the separate characters of the two forces to become apparent. At "low" temperatures—less than about10¹⁵ K, a range that includes almost everything

The key predictions of the electroweak theory were experimentally verified in the 1970s, winning the theory's originators the 1979 Nobel Prize in physics. The GUTs have not yet been experimentally verified (or refuted), in large part because of the extremely high energies that must be reached in order to observe their predictions.

An important idea that has arisen from the realization that the strong and the electroweak forces can be unified is the notion of supersymmetry. It extends the idea of symmetry between fundamental forces to place all particles—those that feel forces (such as protons and electrons), and those that transmit those forces (such as photons and gluons)—on an equal footing. One particularly important prediction of this theory is that all particles should have so-called supersymmetric partners—extra particles that must exist in order for the theory to remain self-consistent. None of these new particles has ever been detected, yet many physicists are convinced of the theory's essential correctness. Experiments are planned that may soon provide evidence for supersymmetric partners to some of the known elementary particles.

Of particular interest to astronomers, these new particles, if they exist, would have been produced in abundance in the Big Bang and should still be around today. They are also expected to be very massive—at least a thousand times heavier than a proton. These so-called supersymmetric relics are among the current leading candidates for the dark matter in the universe (although it must be admitted that recent experimental failures to detect them has dampened some astronomers' early enthusiasm).

Efforts to include gravity within this picture have so far been unsuccessful. Gravitation has not yet been incorporated into a single "SuperGUT," in which all the fundamental forces are united. Some theoretical efforts to merge gravity with the other forces have tried to fit gravity into the quantum world by postulating extra particles—called gravitons—that transmit the gravitational force. However, this is a very different view of gravity from the geometric picture embodied in Einstein's general relativity, and combining the two into a consistent theory of quantum gravity has proved very difficult. Alternative approaches start from the geometric view and attempt to explain the basic forces of nature in terms of additional curved dimensions of spacetime. They, too, encounter serious problems. A theory currently under very active investigation seeks to interpret all particles and forces in terms of particular modes of vibration of submicroscopic objects known as strings. Some theorists feel that this approach, although very complex in its mathematical details, may offer the best hope of unifying the forces of nature.

However, at present, none of these theories has succeeded in making any definite statement about conditions in the very early universe. A complete theory of quantum gravity continues to elude researchers.