In modern physics, the underlying substance that is believed to unify matter and energy is called a “field”.
A field is a physical entity that fills space and time, and it can vary in strength and direction at different points in space and time. Fields can be thought of as the medium through which forces and interactions between particles occur.
According to quantum field theory, all particles and forces in the universe arise from the behaviour of underlying fields, which interact with each other and with particles to give rise to the properties and behaviour of matter and energy.
In this view, matter and energy are simply different manifestations of the same underlying fields, and they can be transformed into each other through various processes.