Gravitational Lensing is an optical phenomenon created by the bending of the light from distant galaxies
by Dark Matter

Gravitational Lensing provides a way for scientists to measure Dark Matter

The light from distant galaxies is bent by the curvature of space due to the stars and Dark Matter. The more Dark Matter there is the more bending occurs. By measuring the bending scientists can get an idea how much Dark Matter is around and whether it is enough to stop BIG RIP that may result in a BIG CHILL or BIG CRUNCH. Gravitational Lensing is a useful way to measure Dark Matter.