Dark matter is not only the most abundant form of matter in the Universe, it's also the most mysterious. Whereas all the other particles we know of — atoms, neutrinos, photons, antimatter and all the other particles in the Standard Model — interact through at least one of the known quantum forces, dark matter appears to interact through gravity alone.
According to many, it would be better to have called it invisible matter, rather than dark matter. It not only doesn't emit or absorb light, but it doesn't interact with any of the known, directly detectable particles through the electromagnetic, strong, or weak nuclear forces. The most sought after dark matter candidate is the WIMP: the Weakly Interacting Massive Particle. The big hope was for a WIMP miracle, a great prediction of supersymmetry.
It's 2019, and that hope is now dashed. Direct detection experiments have thoroughly ruled out the WIMPs we were hoping for.
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