The Universe is about 13 thousand million years old.  Its birth certificate, however, has never been found. 

What does seem certain is that the Universe began with a singularity- a miniscule point that contained the entire universe.  It was at once massive and microscopic, which means that the laws of the huge- General Relativity- and the laws of the tiny- Quantum Mechanics- momentarily overlapped.  Since Quantum Mechanics is goverened by some dodgy randomness, the universe did not expand uniformly.  Happily, this meant that it became clumpy and allowed stars and galaxies to emerge, which in turn led to us.

The early Universe's was rather hot (to say the least) most of it was radiant energy (light) rather than particles.  The early expansion of the singularity was astonishingly swift and vast under a process known as inflation (suggesting God is a Central Banker), and correspondingly cooled as the energy spread out. 

The universe's temperature was so high that the early protons smashed into each other fast enough to result in a nuclear fusion that converted hydrogen into helium- the same process that takes place in the core of the sun today.  For a few minutes our universe was a hydrogen bomb!. 

Three minutes after the Big Bang the temperature had cooled to a balmy billion degrees- enough to switch off the nuclear fusion so that no more hydrogen was converted into helium.   To this day, the vast majority of the universe still consists of hydrogen and helium, and their relative abundance has not changed much since ... about 71% hydrogen and 27% helium.  The heaver elements comprise less than 2% of the universe and are a sideshow that have emerged in the remaining 13 thousand million years minus 3 minutes.