Interesting to consider. Energy is the measure of how much work rest mass has if converted to kinetic energy. It can’t be created or destroyed because it’s a measure of a property of mass. A rest mass’ potential energy is E = mc2. In fusion explosions .07% of mass is converted into other forms and mass is reduced that much.
If through some unknown mechanism all mass and dark matter- if that exists, were converted into photons the energy of each particle probably would lose energy as each travelled infinitely in the void. Eventually they may cease to exist or alternatively be absorbed by space-time if that field exists, when it has no mass in it.
