Task: Text Categorization
Input (English): Theories that do not include this substance include John Moffat's theory that G over c holds constant over time but neither quantity does individually and one proposed by Bekenstein called TeVeS. Its presence explains the formation of stellar structure through the Jeans instability. It may be accounted for by certain leptons with the opposite helicity from experimental prediction, called "sterile. " One model of this substance suffers from the "cuspy halo problem" and cannot explain the lack of many satellite galaxies a thousandth the size of the Milky Way. One hypothesized example of this substance is the Nambu-Goldstone boson that results from spontaneously broken Peccei-Quinn symmetry; that particle accounts for the theta-bar term in strong CP violation and is called the axion. Its baryonic forms include brown dwarfs and MACHOs. For 10 points, name this hypothetical substance postulated by Fritz Zwicky, which is classified based on its speed as hot, warm, or cold and is detectable only by its gravitational effects, not by emitted radiation.
Output (English): Science