“What is it that breathes fire into the equations and makes a universe for them to describe? Why does the universe go through through all the bother of existing?”

Stephen Hawking

He could have asked, Where did the equations find their origin? His problem with a purely natural option is that the universe is made up of everything that physically exists. A natural/ material / scientific explanation has to involve a physical cause. But a physical cause of any type is by definition part of the universe that is being explained. A more circular argument would be hard to find.