How does the concept of wave$-$particle duality challenge our traditional understanding of the nature of matter and energy in quantum mechanics? Discuss the historical context leading to the development of this concept, including key experiments such as the double$-$slit experiment and the photoelectric effect. Analyze how wave$-$particle duality manifests in various phenomena, such as the behavior of electrons in atoms, the interaction of light with matter, and the formation of matter waves. Furthermore, explore the implications of wave$-$particle duality for our understanding of the fundamental nature of reality, including its role in the formulation of quantum field theories and the search for a unified theory of physics. Finally, evaluate the philosophical implications of wave$-$particle duality, including its challenges to classical notions of determinism, causality, and the observer's role in shaping reality, within the framework of modern physics theories.