One of the great mysteries facing scientists today is the answers to the two following questions:
Positrons (anti-electron) and neutrinos, ghostly particles with no mass or electric charge. Finally, the universe was filled with light. Light consists of particles of zero mass and zero electrical charge known as photons. The number and the average energy of the photons was about the same as for electrons, positrons or neutrinos. These particles were continually being created out of pure energy (original source of all matter in the present-day universe-galaxies, nebulae, stars, planets, earth, etc.).
REFERENCE: Weinberg, S.: The First Three Minutes. A Modern View of the Origin of the Universe. Basic Books, Inc. Publishers, NY 1977.