Question

Is a new theory of light and matter needed to explain what happens at very high energies and temperatures?

Answer 1

Matter grows so hot that it interacts with radiation in unfamiliar ways, and photons of radiation can crash into each other and create new matter.

The distinction between matter and energy grows blurry.

Throw in the added factor of magnetism, and physicists can make only rough guesses about what happens in these hellish settings.