The Y comes from the mass of matter converted in to energy. The mass of the material before the reaction is greater than the mass of its products.
The law you're referring to might be the conservation of energy, but that applies to non-nuclear reactions and is more accurately called the law of conservation of mass-energy. In this case the energy in times the mass at the start is still equal to the energy out times the mass at the end. For the energy to increase, the mass must decrease to produce the Y in your equation.
The law you're referring to might be the conservation of energy, but that applies to non-nuclear reactions and is more accurately called the law of conservation of mass-energy. In this case the energy in times the mass at the start is still equal to the energy out times the mass at the end. For the energy to increase, the mass must decrease to produce the Y in your equation.