Dark Matter
Definition
The invisible stuff outweighing all visible matter 5-to-1 — detected only by its gravity holding galaxies together.
Technical definition
Hypothetical non-luminous matter inferred from galaxy rotation curves, gravitational lensing, and cosmic microwave background measurements. Comprises ~27% of the universe's energy density versus ~5% ordinary matter. Its particle nature is unknown — a major unsolved problem.
Example
Outer stars in spiral galaxies orbit far too fast for the visible mass — dark matter's extra gravity is the leading explanation for why galaxies do not fly apart.
Common confusion
Dark matter is not antimatter, black holes, or ‘dark energy’ — it is a separate mystery. ‘Dark’ just means it does not interact with light; it might not even be matter as we know it.