Gravitational lensing is bending the light towards the large objects (cluster of galaxies, Balckholes) when it travels through the universe. Einstein’s general relativity predicts that space and time can bend by the matter (energy density), and this is one of the observations that used to proves Einstein was right.
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The light bending leads to the warping of light from distant galaxies as the light encounter supermassive galaxies on their path to us. This is called gravitational lensing.
This imagethat shows how this lensing works on a grand scale. A distant galaxy would be seen here on earth directly, if there were no intervening massive cluster to bend the light. But with such a cluster the light from the distant galaxies gets bent into rings and arcs to continue on to the earth.
This is able 1689, 2.2 billion light-years away. It’s one of the most massive galaxy clusters known. The gravity of its trillion stars plus dark matter acts like a two million light-years wide lens in space.
This is another cluster 5.7 billion light-years away. It’s the latest from Hubble on gravitational lensing released in late 2015. This’s foreground galaxy clusters are magnifying the light from the faint galaxies that far behind clusters themselves. It’s the gravitational lensing that allows us to see that far back in time.
without the magnification, these galaxies would be invisible for us