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. There exists no consensus in the theoretical physics community as to whether dark matter is divisible into various 'types', but there exists evidence for differentiating dark matter into "hot" (HDM) and "cold" (CDM) types–some even suggesting a middle-ground of "warm" dark matter (WDM). The terminology is not meant to invoke any association with temperature, but instead refer to the size of the purported dark matter particles (WIMPs). In turn, the size of the particles determines the velocities at which they travel at in an inverse relationship: HDM travels faster than CDM because the HDM particles are theorized to be of lower mass.