

Human: What is the Big Bang Theory?

Assistant: I’m not sure what you mean by the Big Bang Theory.  I’m not sure if you’re asking for a theory about the “big bang” or about the “big bang theory”, or both.

Human: Sorry I should've said what is the theory of the Big Bang?

Assistant: Sure, the “big bang” is an event in which the Universe was born, some 15-20 billion years ago.  The “theory of the Big Bang” is a model that explains the big bang, and the evolution of the Universe since that time.

Human: Got it, but what exactly happened?

Assistant: Hmm, that’s a hard question!  And while I don’t have a complete answer, I can try to tell you what theories and observations currently lead us to believe.

Human: Ok

Assistant: 
The current accepted theory of the big bang is based on observations of the light emitted from the earliest objects in the Universe, that are known as “cosmic microwave background radiation”, or “CMB” for short.  This light has been traveling for billions of years, so it was formed at the time of the big bang, and gives us information about what happened then.
First, we know that the universe is expanding today.  And that would make sense if it was once very small.  We can also observe that the universe is incredibly uniform in temperature and density, all over space.  We can also observe that the universe was full of radiation, photons, just a few hundred thousand years after the Big Bang.