The law of conservation of mass is a feature of classical physics and is not strictly followed, perhaps the most famous example being nuclear reactions, cf. mass defect and E=mc²."Matter did not exist then"
Either matter and energy always existed or these are wrong
Law of Conservation of Mass..
-mass can neither be created nor destroyed,
Law of conservation of energy..
-energy can neither be created nor destroyed
I think the Big Bang model allows for the possibility that matter condensed from radiation, via pair-production.