Representation of the Big Bang and its phases.
The Big Bang Theory is one of the most accepted theories in the scientific world as a result of the appearance of the Universe. According to it, all the matter in the Universe, in a moment, was condensed in a single point, and from this, it expanded exponentially to what we know today. Even if it deals with the origin of the Universe, there is not enough basis today for us to know what happened before that, or even how the physical laws acted in the moments when all the matter and energy was hot and condensed in a small space.
How was it discovered
It was first theorized by the priest Georges Lemaître, who proposed the idea that the Universe had origined from a single point, called the primordial atom, and expanded from there. This theory was named Big Bang by an opponent of he, who would have made an irony with this name, which means "big explosion". Only decades later, Edwin Hubble obtained enough evidence that the Big Bang could actually have happened. From his observations, Hubble discovered through the doppler effect, that galaxies were moving away from each other, as proposed by the Big Bang theory. From this moment on, this infamous theory would have became popular and conquered over the scientists of the time.
Contemporary evidences
Even after Hubble, even more evidences emerged that confirmed the Big Bang, as was general relativity, which correctly predicted this event, and the discovery of the universe's cosmic microwave background. The second test cited was extremely important, since it gave the Big Bang a very solid basis, having records of microwave radiation emitted by the stars approximately 13 billion years ago (age of the Universe).
Cosmic Calendar
A very interesting experience is the so-called cosmic calendar, which the entire history of the Universe is divided into 365 days, and each event occurs in one day, which gives a certain sense of the gigantic time between the appearance of humans, and the event of the Big Bang, for example. Below is a demonstration of this cosmic calendar presented by the documentary Cosmos.
Bibliography
https://www.ufjf.br/
http://www.cbpf.br/
http://portal.mec.gov.br/