### Instruction: "Big Bang": what does it mean?
### Response: The Big Bang theory, which is how astronomers explain the origin of the universe, holds that the universe as we know it began billions of years ago in a huge explosion. At the time of this explosion, all the matter that exists in the universe today was concentrated at one point.

From the moment of the explosion, matter began to expand and is still expanding today. That is why the Big Bang theory includes the theory of the expanding universe. In addition to expanding, matter began to form more complex structures, from atoms and molecules to the living organisms we know today.

The name "Big Bang" was first used by physicist Fred Hoyle in 1949, who disagreed with this theory, preferring the steady-state model of the universe.  The idea of a big bang seemed absurd to Hoyle.