The big bang theory states that the universe was formed as the result of a huge explosion.   how did the universe begin, according to the theory?

In simpler terms, the Big Bang theory says the universe as we know it started with a small singularity, then inflated over the next 13.8 billion years to the cosmos that we know today. In the first second after the universe began, the surrounding temperature was about 10 billion degrees Fahrenheit (5.5 billion Celsius). The cosmos contained a vast array of fundamental particles such as neutrons, electrons and protons. These decayed or combined as the universe got cooler. Over time, however, the free electrons met up with nuclei and created neutral atoms. This allowed light to shine through about 380,000 years after the Big Bang.