... "The law of conservation of matter or principle of matter conservation states that the mass of an object or collection of objects never changes over time, no matter how the constituent parts rearrange themselves.
The mass can neither be created nor destroyed."
That's from classical physics. You must account today for general relativity and quantum physics, resulting a the e=mc^2 equation and hold that the sum of mass and energy are thus related and bound in a closed system.
In the beginning there was no energy, no mass, no matter etc. Where did all this energy come from?