Hypothetical particles are theoretical entities in physics that scientists propose to explain certain phenomena or gaps in our understanding of the universe. These particles have not yet been observed directly, but they are predicted by various theories, such as string theory or supersymmetry. Examples include gravitons, which are thought to mediate the force of gravity, and axions, which may help solve the mystery of dark matter. Researchers study these particles to explore fundamental questions about the nature of matter and energy. By investigating hypothetical particles, scientists hope to uncover new insights into the workings of the universe and potentially discover new forces or interactions that govern everything around us.