After many calculations and experiments in physics laboratories, it is believed that at the beginning of time, the universe was microscopic in size and contained a tremendous amount of energy.
At the dawn of time, there was therefore no galaxy, no star, no planet, no atom, no gas, nothing. The whole universe was in a volume billions of times smaller than an atom, super hot and super dense and containing only energy. It must have looked like a kind of light, but its energy was so high that it would kill anyone in a second, like gamma rays but more intense.
In fact, all this energy was needed because it gave birth to all the atoms, all the matter and therefore to all the stars and galaxies we observe today in the universe.
In the beginning, then, all of a sudden, it is thought that there was a burst of energy, like a larger wave than the others. She gave birth to our universe during a phenomenon that looks like an explosion and is called the Big Bang. This energy was so important that it created the material that gave birth to the universe!
At first there were only elementary particles. Over time, they slowly combined to form the first atoms and molecules.
For a few hundred thousand years, the universe was not dark, but rather bright because of the omnipresent energy. Then, when it was large enough and cold, it became transparent like today. But in the beginning there were no stars or galaxies yet.
Then, the first atoms and molecules associated to form clouds of gas gave birth to the stars. The stars then gathered to form the galaxies.
After 10 billion years, the Sun was born in a cloud of gas in one of these galaxies, the Milky Way. The Earth, the Moon and other planets formed afterwards and when the Earth was cold enough, about 800,000 years later, life developed there. It took almost 4 billion years for the man to appear, then one day your parents were born and here you are! Thus, since the beginning of the universe it has passed about 14 billion years.