According to the current understanding of the Big Bang theory, the early moments of the universe were characterized by extremely high temperatures and densities. The universe was initially in a state of extreme energy and, as it expanded and cooled, particles and matter began to form.
In the first fractions of a second after the Big Bang, the energy of the universe was primarily in the form of high-energy particles, such as quarks, gluons, and other elementary particles. As the universe continued to expand and cool, these particles combined and interacted to form protons, neutrons, and electrons, which are the building blocks of ordinary matter.
The process by which energy is converted into matter is known as particle creation, and it can occur under certain extreme conditions. In the early universe, the high energy densities allowed for the creation of particles and antiparticles from the available energy. However, due to a process called annihilation, particles and antiparticles would often collide and annihilate each other, converting their mass-energy back into pure energy.
As the universe continued to expand and cool further, the density and temperature dropped to a point where particle creation and annihilation became less frequent. Eventually, the universe reached a stage where the energy was no longer sufficient to create new particles and the existing particles could no longer find their antimatter counterparts for annihilation. At this point, a significant portion of the initial energy had been converted into matter, leading to the formation of protons, neutrons, and electrons, which later combined to form atoms and more complex structures.
In summary, during the early moments of the universe after the Big Bang, there was a conversion of energy into matter through processes such as particle creation. However, it's important to note that the precise details of these processes and the exact nature of the early universe are still active areas of research and are subject to ongoing study and refinement within the field of cosmology.