If energy cannot be created or destroyed, the universe's total energy must be a fixed value. But because the cosmos is physically larger today than yesterday, it breaks this rule every second.
To understand why, we have to look at the work of mathematician Emmy Noether. In 1915, she proved a mathematical theorem showing that every conservation law in physics is tied to a specific symmetry in nature. The conservation of energy is explicitly tied to time-translation symmetry. This symmetry means that the underlying background of a system does not change over time; an experiment performed today will yield the exact same results if performed tomorrow. Because time-translation symmetry holds true for everyday interactions, energy is perfectly conserved when a ball bounces or a car engine burns fuel.
The universe itself is the major exception to this rule. As the fabric of space stretches, its fundamental geometry changes. It does not possess time-translation symmetry on a global scale. Without that symmetry, the law of conservation of energy simply does not apply.
We can observe this broken rule in two spectacular ways. The first is cosmological redshift. As light from distant galaxies travels toward Earth, the expansion of space stretches the photons. Their wavelength increases, which means their energy drops. The energy lost by these stretching photons does not transfer into heat, gravity, or some hidden particle. It simply ceases to exist.
The second observation involves dark energy, the mysterious force accelerating cosmic expansion. Dark energy has a constant density, meaning a specific volume of space always contains the exact same amount of it. As the universe expands and creates more volume, the total amount of dark energy in the universe increases. The universe continually generates new energy out of the expanding void.