According to the theory of the Big Bang, an enormous singular explosion created the universe 13.7 billion years ago. It also caused the expansion and cooling-down of the universe which continues until today .
The theory, developed around 1920, has actually been adapted by most scientists these days. It replaced the contemporary image of a static, homogenous, infinite universe (even Einstein adapted it).
Since then, a number of observations have been made that support the theory of the Big Bang (as for example the discovery of the cosmic background radiation or the “echo” of the Big Bang, which was mentioned by the theory).
However, scientists are still struggling to explain what really happened in the moment of the explosion – and especially what was there before, if you can even speak about “before”.
According to the string theory, the formation of our universe was caused by the collision of two parallel universes. Following another theory, the universe is expanding infinitely. Still another theory says that the universe expanded, then contracted again … like infinite breath.