What is dark matter made of?

Astronomical observations show that about 27% of the energy density of the universe is in some form of non-luminous matter that interacts gravitationally but apparently not (or barely) electromagnetically. Its particle nature is unknown.

Leading candidates include weakly interacting massive particles (WIMPs), axions, sterile neutrinos and primordial black holes. Decades of direct-detection experiments have produced increasingly stringent null results without identifying the constituents.