Dark matter is an invisible form of matter from which, scientists now believe, 90% of the Universe is composed.
Dark matter is invisible: it gets its name because, since it doesn't emit, or reflect, light it cannot be seen, even by telescopes.
We know dark matter is there, only by its gravitational effects on the parts of the Universe we can see.
For example, clouds of stars and gas in galaxies stay together, even though they don't possess enough gravity to explain why. Only the presence of more mass, as yet unseen by our technology, explains this.