..and that is the end of your tour of space! Any questions?
Scientists agree that billions of years ago, an initial explosion occurred. This was known as the Big Bang. It was said to be the beginning of the formation of the universe.
The explosion became a theory based on three pieces of evidence... One piece is cosmic background radiation. This is electromagnetic radiation leftover from the Big Bang.
As the Universe expands and galaxies move apart, the wavelength of light emitted from those galaxies is stretched. This shifts the light toward the red end of the spectrum and is called “red-shift”. The more faint a galaxy, the more rapidly it is moving away from Earth.
This process of light element formation in the early universe is called “Big Bang nucleosynthesis”. The predicted abundance of deuterium, helium and lithium depends on the density of ordinary matter in the early universe.
From then on and continuing today, the universe expands.