Spacetime arrives when supergravity separates into the combined nuclear forces (strong, weak, electromagnetic) and gravitation. Matter makes its first appearance during this era as a composite form called Grand Unified Theory or GUT matter. GUT matter is a combination of what will become leptons, quarks and photons. In other words, it contains all the superpositions of future normal matter. But, during the GUT era, it is too hot and violent for matter to survive in the form of leptons and quarks.

Why can't matter remain stable at this point in the Universe's evolution? This involves the concept of equilibrium, the balance between particle creation and annihilation.

During pair production, energy is converted directly into mass in the form of a matter and anti-matter particle pair. The simplest particles are, of course, leptons such as an electron/positron pair. However, in high energy regimes, such as the early Universe, the conversion from energy to mass is unstable compared to the more probable mass to energy conversion (because the created mass must be so high in mass to match the energy used). In other words, when temperatures are high, matter is unstable and energy is stable.

Any matter that forms in the early Universe quickly collides with other matter or energy and is converted back into energy. The matter is in equilibrium with the surrounding energy and at this time the Universe is energy or radiation-dominated.

The type of matter that is created is dependent on the energy of its surroundings. Since the temperatures are so high in the early Universe, only very massive matter (= high energy) can form. However, massive particles are also unstable particles. As the Universe expands and cools, more stable, less massive forms of matter form.

As the Universe expands, matter is able to exist for longer periods of time without being broken down by energy. Eventually quarks and leptons are free to combine and form protons, neutrons and atoms, the ordinary matter of today.

Quarks and Leptons :

After GUT matter forms, the next phase is for GUT matter to decay into lepton and quark matter. Lepton matter will become our old friends the electron and neutrino (and their anti-particles). But quark matter is unusual because of the property of quark confinement.

Quarks can never be found in isolation because the strong force becomes stronger with distance. Any attempt to separate pairs or triplets of quarks requires large amounts of energy, which are used to produce new groups of quarks.

With so much energy available in the early Universe, the endresult is a runaway production of quark and anti-quark pairs. Trillions of times the amounts we currently see in the Universe. The resulting soup of quark pairs will eventually suffer massive annihilation of its matter and anti-matter sides as soon as the Universe expands and cools sufficiently for quark production to stop.

Notice that quark pairs are more stable than triplets, so that most of the quark production is done in pairs. Later, pairs will interact to form triplets, which are called baryons.