dark-matter candidate particles

We are fairly sure dark matter exists, but we do not know what it is made of. So physicists have drawn up a list of suspects — hypothetical particles that would behave the right way: they must be electrically neutral (so they give off no light, which is what 'dark' means), they must be stable enough to survive since the early universe, and they must barely interact with ordinary matter, so they can drift through galaxies feeling only gravity. None of the particles in the Standard Model fits the bill, so every candidate is a proposed new particle.

Two candidates have dominated the search for decades. The first is the WIMP, short for Weakly Interacting Massive Particle: a heavy particle (perhaps tens or hundreds of times the mass of a proton) that interacts through gravity and possibly the weak force. WIMPs are attractive because, if they exist, the early universe would have naturally produced just about the right amount through a process called thermal freeze-out — a coincidence so neat it earned the nickname the 'WIMP miracle.' The second is the axion, an extremely light particle originally invented to fix an unrelated puzzle in the strong force, which would also make a fine cold, dark, invisible filling for the cosmos. Other ideas include sterile neutrinos and hidden 'dark photons.'

The honest situation is that none of these has been found. Decades of increasingly sensitive experiments have searched for WIMPs and seen nothing, steadily ruling out the simplest versions; axion searches are now reaching interesting sensitivity but have also come up empty. This matters because dark matter is the clearest evidence we have that the Standard Model is incomplete: there is a particle (or particles) out there making up most of the matter in the universe that our best theory does not include, and finding it would be one of the great discoveries of physics.