The idea that mass bends light that travels near it comes from Einstein's theory of general
relativity. In fact, Einstein first achieved worldwide fame in 1919 because another physicist, Arthur
Eddington, observed light being bent by the Sun, confirming the existence of this phenomenon.
The amount by which even a large cluster of galaxies
bends light is typically only very small, much less than one
degree. However, in the video, we have exaggerated the
effect in order to highlight it.
Clusters of Galaxies
Gravitational lensing allows physicists to study dark matter
on the scale of clusters of galaxies, that involve large
numbers of individual galaxies, as in Figure 15. They have
found that the ratio of dark matter to stars and hydrogen
gas on this scale is significantly greater than in
individual galaxies.
Some gravitational lensing observations are difficult, if
not impossible, to explain without dark matter and many
physicists think that gravitational lensing provides the
strongest evidence for the existence of this
elusive material.
Furthermore, as gravitational lensing is a feature of
Einstein's theory of general relativity and not Newton's
theory of universal gravitation, it provides evidence for the
presence of dark matter that is independent of evidence
from the orbital speeds of stars within galaxies.
