Does energy or matter ever disappear? Explain.

The law of Conservation of Energy states that "Energy cannot be created or destroyed.". In other words, the total amount of energy in the universe never changes, although it may change from one form to another. Energy never disappears, but it does change form.

The first law of thermodynamics doesn't actually specify that matter can neither be created nor destroyed, but instead that the total amount of energy in a closed system cannot be created nor destroyed (though it can be changed from one form to another).