Two Kinds Of Gravitational Lenses Both Reveal Dark Matter
“There needs to be more mass than normal matter alone to explain what we see. Some novel, exotic, invisible “dark matter” is the leading theoretical idea. It’s a radical proposition to presume dark matter’s existence, but gravitational lenses can reveal it. In Einstein’s General Relativity, the presence of mass curves the fabric of spacetime. A large collection of foreground mass distorts the background light through the process of gravitational lensing.”
When we look out at the light-emitting objects in the Universe, they get affected by the entirety of the matter that they pass through along the line-of-sight, as that light travels from the source to our eyes. If there are large collections of mass along that line-of-sight, that mass will bend the fabric of spacetime, creating an effect known as gravitational lensing.