Heavy things in space bend the path of light by warping the fabric of space itself.

Imagine space is a giant trampoline. If you drop a heavy bowling ball onto it, the fabric stretches and sinks. That is what massive things like clusters of galaxies do to space. When light from a distant star travels past this heavy object, it cannot go in a straight line. Instead, it follows the curves in space, bending like light passing through a glass lens. Albert Einstein predicted this, and we first saw it happen during a solar eclipse in 1919.