Because scientists can't see dark matter directly, they have found other ways to investigate it. We can use indirect ways to study things, like looking at a shadow and making an educated guess about what's casting the shadow. One way scientists indirectly study dark matter is by using gravitational lensing.
Light going through a gravitational lens is similar to light going through an optical lens: It gets bent. When light from distant stars passes through a galaxy or cluster, the gravity of the matter present in the galaxy or cluster causes the light to bend. As a result, the light looks like it is coming from somewhere else rather than from its actual origin. The amount of bending helps scientists learn about the dark matter present.