Gravitational lensing

In Einstein’s General Relativity, matter and energy bend and distort spacetime due to gravity. When light passes through that distorted region, bending and magnification ensue. It behaves similarly to an optical lens, but powered by gravity: a gravitational lens.

With strong lensing, where clear images are seen, cluster and individual galaxies show gravitational lensing. When the observer, lens, and background objects all align, spectacular features such as arcs, multiple images and even perfect Einstein Rings become possible. A distant massive, compact galaxy was found by JWST behaving as a gravitational lens.

With weak lensing, the distortions of background sources are much smaller and can only be detected by analyzing large numbers of sources in a statistical way to find coherent distortions. The mass distribution and background distribution of dark matter can then be reconstructed. A large number of galaxies need to be viewed in this manner, as well as the cosmic microwave background.

Instead of positive weak lensing that reveals locations of positive mass dark clusters, "negative mass clumps" can be located using negative weak lensing, This allows solutions like traversable wormholes stabilized by exotic matter of negative energy density. Invisible negative mass is an alternative interpretation to dark matter, which classically has a positive mass.

With microlensing, no distortion in shape can be seen but the amount of light received from a background object changes in time. The lensing object may be stars in the Milky Way, with the background source being stars in a remote galaxy. So a star in a distant galaxy can act as a microlens and magnify another star much farther away.