In the distant past most people accepted their religion's explanation of how the universe began. As the world became more interested in science in the 18th and 19th centuries, however, religion alone couldn't always explain the new discoveries being made. In the 1920s the Big Bang theory was proposed as a possible scientific explanation for the creation of the universe.
It was first proposed by Alexander Friedman, a Russian mathematician in 1922 and expanded upon in 1927 by Georges-Henri Lemaitre. He was a Belgian physicist, who was also a Catholic priest. Scientists such as Edwin Hubble have made detailed observations and measurements that support and develop this theory.
Put simply, around 14 billion years ago, all matter and energy in the universe was at a point of infinite density and temperature. It then expanded rapidly, and eventually stars, galaxies and planets formed. This expansion was the beginning of time and continues to this day. The Big Bang theory is supported by evidence that space is expanding, including the redshift of light from distant galaxies and the existence of cosmic background radiation in all directions.
It is thought the Earth was formed around 4.6 billion years ago from dust and gas left after the Sun formed.
As the Earth gradually cooled, creating conditions in which life was possible, living things appeared on the new planet Earth.
Not all scientists agree with the Big Bang theory, but many of their objections are to do with the details within the process, not the underlying principle that it happened. Similarly, there are different hypotheses about how life began on Earth. These can be tested, but scientists cannot be sure which are correct because it happened long ago.