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Hot dark matter is a theoretical form of dark matter which consists of particles that travel with ultrarelativistic velocities.
Dark matter is a form of matter that neither emits nor absorbs light. Within physics, this behavior is characterized by dark matter not interacting with electromagnetic radiation, hence making it dark and rendering it undetectable via conventional instruments in physics. Data from galaxy rotation curves indicate that approximately 80% of the mass of a galaxy cannot be seen, forcing researchers to innovate ways that indirectly detect it through dark matter's effects on gravitational fluctuations. As we shall see below, it is useful to differentiate dark matter into "hot" and "cold" types–some even suggesting a middle-ground of "warm" dark matter. The terminology refers to the mass of the dark matter particles : HDM travels faster than CDM because the HDM particles are theorized to be of lower mass.