In simple terms, what is the Big Bang Theory?
In simple terms, the Big Bang Theory is the leading explanation of how the first moments of the Universe; the beginning of the Universe
See the explanation for less-simple terms.
Because scientists don't yet have the technology of looking back through time to the moments of the Big Bang, much of their explanations are found through mathematical formulas and models. But scientists can, however, see the echo of the Big Bang through cosmic microwave backgrounds.
In the first second after the birth of the Universe, the surrounding area was around 5.5 billion degrees Celcius (10 billion Ferenheight). At this time, there was a vast amount of particles such as protons, neutrons, and electrons in the cosmos which later decayed and combined as the universe cooled down forming many elements, some of them being the elements we know of today.