The Big Bang is a scientific theory that tries to explain the first moments of the Universe. This is the beginning of the expansion of the Universe, about 13.7 billion years ago, when the Universe was extremely dense and hot.
This expression means "Grand Boum" in French, but it is the English expression that is used. This is not the beginning of the Universe as we often say, because we do not know what was before (or even if there was a before). The Big Bang is a scientific theory partly confirmed by certain astronomical observations. If we know that the universe was initially hot and very dense, the theory does not say what is at the origin of the Big Bang. It is therefore different from the creation of the world (religious hypothesis).
This moment marks the beginning of the expansion and expansion of the Universe. The Big Bang is not an explosion: it is the totality of the space that expands, and it is not contained in another larger space. The term Big Bang was used for the first time in 1950. At first, it was a physicist, Fred Hoyle (wp), who created this term to make fun of this new cosmological theory. This physicist believed, like many other scientists of that time, that the Universe had always been the same.
No observation can tell directly what happened 13.8 billion years ago but we can still observe some phenomena that occurred 380 000 years after the Big Bang.