How is m conserved?? x?

More accurately, m and energy are conserved. E=mc^2 only proved that m can be converted to energy and vice versa. Now, scientists consider m to be just another form of energy. The conservation of energy is a fact. You can't lose or gain energy, only convert it into different forms. For clic Newtonian physics, we say that m is conserved because we have no way of converting mass into pure energy.