Fritz Zwicky first proposed in 1934, and Vera Rubin showed in 1975 that roughly 90% of the mass of any given galaxy is found in an invisible sphere with more than 10 times the radius of the visible galaxy.

The completely unknown particles that make up 85% of the mass of the universe must be weakly interacting— could be fermions with small mass, but maybe not so small as neutrinos, which have a mass so tiny no one has been able to figure out how to measure it. Or they could be bosons with mass about the same as neutrinos. These particles of dark matter are not in, and were not predicted by, the Standard Model, and it is very difficult to detect one here on the surface of the earth. A major effort will be needed to get an idea of their physical properties. They're fermions or bosons, and came out of the Big Bang along with photons, neutrinos, electrons, neutrons and protons... but what are they?  WHY are they?  How can we capture one in a particle detector?