What particle (or object) makes up dark matter?
Galaxy rotation curves, gravitational lensing, the Bullet Cluster and the CMB all require about five times more gravitating mass than visible baryons provide. Whether dark matter is a weakly interacting particle, an axion, a primordial black hole population, or something else remains unidentified.
Why it is hard
Decades of direct-detection experiments (Xenon, LUX-ZEPLIN, PandaX) have found nothing, pushing WIMP cross-sections below the 'neutrino floor'. The allowed mass range spans ~90 orders of magnitude, from ultralight axions to asteroid-mass primordial black holes — no single experiment covers it.
Current evidence
Gravitational evidence is overwhelming and consistent across scales, but no non-gravitational detection exists. Small-scale puzzles like the core–cusp problem and missing-satellites concerns motivate self-interacting or ultralight dark matter models.
Possible approaches
Next-generation direct detectors at the neutrino floor, axion haloscopes (ADMX) and helioscopes, indirect searches for annihilation products with CTAO and Fermi, micro-lensing constraints on primordial black holes, and precision tests of gravity on small scales.
Related topics
Sources
- Dark Matter — NASA Science
- Hubble Dark Matter — Bullet Cluster — NASA Science
- Planck and the cosmic microwave background — European Space Agency
- CTAO Technology — How CTAO works — Cherenkov Telescope Array Observatory