The Big Bang theory is used to explain the origins of the universe. Why is this called a theory and NOT a law?

It's called a theory because it has not been proven yet. so we can not claim it to be a fact without proof.

There is no concrete evidence.

Explanation: If we could do an experiment and create the Big Bang and watch it happen and study it, it would be law. It's for the same reason that evolution is considered a theory. We are not watching it take place and there is no eye witness account of evolution happening because evolution happens over millions and millions of years. If we could sit here and watch the universe be created without need for simulations or theories, then it would be law.