Why does light bend near a gravitational field?

The gravitational lensing effect is one of Albert Einstein’s predictions on the general theory of relativity. To put it in simple terms, lensing is the bending of light by mass. 
Let’s assume a ray of light passes in close proximity to an object (galaxy or cluster) with a huge mass. The gravitational field of such objects is powerful enough to bend light and refocus it. The more massive the object is, the greater the light bending. There is however something massive enough surrounding all galaxies, and it’s called dark matter. We cannot see it, but we know it’s there, because it has mass. However, dark matter does not interact at all with light (hence the dark/invisible part). It can, however, bend light because it has mass and gravitation. When we look through our telescopes, we see galaxies or clusters that might appear distorted, yet the light from these systems may be bent by the gravitational power of dark matter (or other galaxies, clusters and even stars), hence the lensing effect. In short, the gravitational bending is the distorted image of a galaxy or galaxy cluster we see though our telescopes and it’s all because of this lensing effect that makes galaxies and clusters appear elongated and distorted. Gravitational lensing also has its upsides as it can show the amount of dark matter present in space between our telescopes and the galaxies or cluster we observe (although we are still pioneers on the subject). 
I’ve attached a picture of NGC 1300 (>60 million light years away from Earth), a bared spiral galaxy from the Eridanus Cluster, surrounded by enormous amounts of dark matter, that might also explain the visible fracture of one of its spiraling arms and the distorted shape of the galaxy itself.

A while ago I published a post about a Katherine Freese’s book, The Cosmic Cocktail: Three Parts Dark Matter. Take a look at this book as it explains gravitational lensing, dark matter and dark stars among many other interesting subjects. – Roman Alexander

(The question was originally asked by Nityananda Padhi)