Big Bang Theory video above.

Scientists believe that the big bang is how the universe started. The universe was at a hot, dense state that expanded rapidly. The rapid expansion of the universe caused it to cooled and resulted in the ever expanding universe. Approximately 13.75 billion years ago the expansion happened, and is known as the age of the universe. It is a well tested theory and accepted in the scientific community.
When the big bang occured the first gas produced was hydrogen along with traces of helium and lithium. During the earliest stages of the big bang, in most common models,
the universe had an incredibly high energy density and huge temperatures and pressures and was very rapidly expanding and cooling. A few minutes into the expansion the temperature was about a billion kelvin and the density was about the same as air. The protons and neutrons combined. Over a long period of time, gases started to combine and form planets that we have today.