Ever heard of the Big Bang? No, not the TV show. The beginning of the Universe as we know it. 

The Big Bang theory is the prevailing cosmological model explaining the existence of the observable universe from the earliest known periods through its subsequent large-scale evolution. The model describes how the universe expanded from an initial state of high density and temperature,  and offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, and large-scale structure.

Crucially, the theory is compatible with Hubble–Lemaître law—the observation that the farther away a galaxy is, the faster it is moving away from Earth. Extrapolating this cosmic expansion backwards in time using the known laws of physics, the theory describes an increasingly concentrated cosmos preceded by a singularity in which space and time lose meaning (typically named “the Big Bang singularity”). Detailed measurements of the expansion rate of the universe place the Big Bang singularity at around 13.8 billion years ago, which is thus considered the age of the universe. 

Timeline

The first second after the Big Bang, the entire universe was a soup of subatomic particles, superheated to 10 billion degrees. In that first second, amazing things happened: The force of gravity separated out from the electronuclear force and was joined soon thereafter by the electromagnetic force. The universe changed from being a hot soup of quarks and gluons (elementary particles), and protons and neutrons began to form. At the ripe old age of one second, the newborn universe was cool enough that it began forming deuterium (a form of hydrogen) and helium-3. At this point, the newborn universe had doubled in size at least ninety times!

Over the next three minutes, the infant universe continued to cool down and expand, and the creation of the first elements continued. 

For the next 370,000 years, the universe continued its expansion. But it was a dark place, too hot for any light to shine. There existed only a dense plasma, an opaque hot soup that blocked and scattered light. The universe was essentially a fog. 

The next big change in the universe came during the era of recombination, which occurred when matter cooled enough to form atoms. The result was a transparent gas through which the original flash of light from the Big Bang could finally travel. We see that flash today as a faint, all-encompassing, distant glow called the cosmic microwave background radiation (sometimes shortened to CMB or CMBR). The universe was leaving its cosmic dark ages behind. Gas clouds condensed under their own self-gravity (possibly helped along by the gravitational influence of dark matter) to form the first stars. These stars energized (or ionized) the remaining gas around them, lighting up the universe even more. This period is called the Epoch of Reionization.

From the Big Bang to You 

Pre–Big Bang: quantum density fluctuations

Pre–Big Bang: cosmic inflation

13.8 billion years ago: the Big Bang

13.4 billion years ago: the first stars and galaxies 

11 billion years ago: the Milky Way Galaxy starts to form 

5 billion years ago: the Sun begins to form, along with the planets 

3.8 billion years ago: the first life appears on Earth 

2.3 million years ago: the first humans appear Modern time: you were born

https://en.wikipedia.org/wiki/Big_Bang
https://science.nasa.gov/astrophysics/focus-areas/what-powered-the-big-bang