The total mass or total energy of the closed system that is the Universe is constant and has been constant since the beginning.
It is true that matter cannot be created or destroyed. We have to understand 2 laws which are essentially the same. The conservation of mass states that in a closed system during any reaction total mass must be preserved so the total mass of the reactants or the starting materials of the reaction will be the same as the mass of the products.
The adapted version for thermodynamics which is the First Law of Thermodynamics states that The total energy of an isolated system is constant so it may transform to other forms freely but cannot be created nor destroyed it is a constant.
The Universe is an isolated system and even with it being an expanding Universe it means that at the beginning of the Universe whatever form of Energy was present in the small point after started the the energy present is the same as the energy present now. And mass is included in that too.
The Big Bang isn’t an explosion in itself it wasn’t an explosion that created space it was the beginning of an expansion which created space-time as we know it.