Dark matter is the mysterious stuff that holds the galaxies together and is an essential part of the Standard Model of Cosmology — the very successful theory explaining the Universe from the Big Bang 13.8 billion years ago to today. Evidence of dark matter is most obvious in the rigid-rotor motion of galaxies versus the multi-velocity rotation of the solar system.
In a spiral galaxy, stars far from the center orbit just as fast as stars close to it — like a rigid spinning disk. But if only the visible matter (stars and gas) were generating gravity, we'd expect outer stars to orbit much more slowly, the way the outer planets of our solar system orbit the Sun slowly. This discrepancy is one of the strongest pieces of evidence that a vast, invisible form of matter pervades galaxies and dominates their gravitational structure.
The Standard Model of Cosmology — also called ΛCDM (Lambda Cold Dark Matter) — accounts for this by proposing that roughly 85% of the mass of the universe is made of dark matter. This model has been spectacularly successful at predicting the large-scale structure of the universe, the cosmic microwave background, and the formation of galaxies. Yet the identity of dark matter remains unknown.
Candidates range from hypothetical weakly interacting massive particles (WIMPs) to axions, sterile neutrinos, primordial black holes, and — as explored by this collaboration — Magnetized Quark Nuggets (MQNs). Unlike most candidates, MQNs are predicted to be macroscopic objects with a strong magnetic dipole moment, which makes them potentially detectable and, ultimately, collectible.