Dark matter
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Unsolved problem in physics:
What is dark matter? How was it generated?
In astronomy, dark matter is a hypothetical form of matter that appears not to interact with light or the electromagnetic field. Wikipedia
Light bosons: quantum chromodynamics axions
macroscopic: primordial black holes
modified gravity (MOG): modified Newtonian dynamics (MoND)
neutrinos: Standard Model
other particles: weakly interacting massive particle
weak scale: supersymmetry