What is dark matter?

Scientists don't always know how to explain dark matter. There is more unknown than known. Dark matter was found in the 50's. Once, scientists saw that two stars in a spiral galaxy were moving at the same velocity. This proved the galaxy had more mass than could be seen by the human eye.
Dark matter composes 27% of the universe. They call it dark, because it's not made the same way than stars and planets. It isn't a dark cloud of small matter. Nor is it a large galaxy-sized black hole. It does not emit light of energy, which makes it hard to detect. Scientists believe it's made out of exotic particles like axions or Weakly Interacting Massive Particles (WIMP's).
Still to this day, dark matter can only be detected by it's gravitational effects. And scientists all over the world are still trying to discover more about this mystery. Space still has a lot to teach us...