As they collide, a force develops between them, as their motion decreases. Kinetic energy is converted to force. If force is interconvertible with motion, as Newton's laws indicated, then force is a form of energy. It's unimaginable, because physicists do not know what force is. All they know about force is what it does.
Theory indicates that the source of energy is a medium below the level of molecules.
There must be at least one, maybe only one, medium at the subatomic level, because electromagnetic waves require such a medium. Only the Michelson-Morely experiment indicates otherwise, and it does not rise to the level of valid science by any criteria. The mystics of the eastern religions describe what they see at the subatomic level, and they describe it as a medium of energy.