Pictured above is the Bullet Cluster, an impressive display of two colliding clusters of galaxies. Normal matter is represented by red, while dark matter (inferred from gravitational lensing) is represented by blue.

Dark matter doesn't emit or reflect enough electromagnetic radiation to be detected. One way to infer whether it's there is through the gravitational effect it has on visible matter surrounding it.

Gravitational lensing isn't the only proof that dark matter exists. Galaxies have been observed moving faster than they should when judging by the amount of normal matter they have. It's as though galaxies have far more mass than we can see.

Although scientists are still figuring out exactly what dark matter is, one thing is for certain: it has more mass than normal matter.

:: Source
:: About gravitational lensing