Theoretical particles are hypothetical entities in physics that help scientists understand the fundamental nature of matter and energy. These particles are not directly observed but are predicted by various theories, such as quantum mechanics and string theory. They play a crucial role in explaining phenomena that cannot be accounted for by known particles. Examples of theoretical particles include the graviton, which is proposed to mediate the force of gravity, and the Higgs boson, which was confirmed in 2012 but was initially a theoretical concept. Studying these particles helps physicists develop a deeper understanding of the universe's underlying principles.