The Big Bang Theory is one explanation of how the Universe may have been created. It's believed that at one point, all of the energy in the Universe today was trapped inside an infinitesimally small volume. This little ball of energy exploded, becoming known as the Big Bang, which created our Universe. No one is really sure when this would have happened, but there is some evidence saying it could have happened anytime between 11 and 18 billion years ago. NASA says that "If we were to look at the Universe on second after the Big Bang, what we would see is a 10-billion degree sea of neutrons, protons, electrons, anti-electrons, photons, and neutrinos" (NASA). The Universe cooled over time until it reached a temperature where electrons would be able to pair up with nuclei to form neutral atoms. The Big Bang was said to give an afterglow, known today as the Comic Background Radiation, which can still be seen today.
https://wmap.gsfc.nasa.gov/universe/bb_cosmo_fluct.html
http://umich.edu/~gs265/bigbang.htm
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