Fundamental Physics
About a quarter of the universe's energy density behaves gravitationally like matter but emits no light. What is it?
Current explanations
Leading candidates are new particles (WIMPs, axions). Alternatives modify gravity (MOND). Gravitational evidence for unseen mass is very strong; particle identification is not.
Research progress
Multiple independent lines (rotation curves, lensing, CMB) confirm the gravitational effect; direct-detection experiments have so far returned null results, tightening constraints.
The gravitational anomalies attributed to dark matter are caused by an as-yet-undetected, weakly-interacting massive particle (or similar) beyond the…
The observed anomalies reflect a breakdown of Newtonian/Einsteinian gravity at low accelerations rather than unseen mass.