What is dark matter?

Dark matter refers to undetectable matter or particles whose presence explains unexpected gravitational effects on galaxies and stars.

Astronomer Fritz Zwicky was the first person to observe the first evidence of dark matter.

Today, due to gravitational lensing of background galaxies by intervening dark matter, we know that dark matter is some form of stuff - particles - that doesn't interact with light accept via gravity, and has a very small cross-section that prevents it from undergoing frequent collisions.