Of this we are ignorant. Mankind has lately begun subdividing the atom into assumed subatomic particles, and increasingly speaking of energy as particles moving rapidly in a wave pattern with varying wave pattern height, width, and speed or what humans call frequency. The fact that energy may be a subatomic particle found in matter, on the move, is increasingly accepted. However, just why it is on the move, and why it moves in waves, is still a bit of a mystery. In discussions of subatomic particle movement, an analogy to the behavior of liquids might help humans relate.
Humans are aware that water can appear to be a solid, when ice, or on the move, when a liquid.
They are also aware that water can take up less space when cold and more when warm, having what is called heat energy stuffed into the same space. This same concept can be applied to what humans call matter and energy, energy simply being particles of matter on the move, and on the move because other particles have been stuffed into the same space. The space becomes crowded, and just as circulation in water is caused in part by temperature differences, just so subatomic particles go on the move to equalize the crowding at the atomic level. Coming out of what is called a big bang, particles are sent in all directions. All are on the move.