Dark matter

Matter that has gravity but does not interact with light, and outweighs the visible sort five to one.

Galaxies rotate too fast for the mass we can see: their outer stars should fly off, and do not. Clusters bend light behind them more strongly than their visible matter can account for. The explanation that fits every observation is that most matter is something that does not emit, absorb or reflect light, and that clumped early enough for galaxies to form in the time available. Decades of experiments — underground detectors, colliders, gamma-ray searches — have not caught a particle. The main alternative, modifying gravity itself, handles individual galaxies well and struggles badly with colliding clusters, where the mass and the visible gas are plainly in different places.