Gravitational lensing

Mass bending light, turning galaxies into arcs and rings and acting as a free telescope.

Mass curves spacetime, so light passing a cluster is deflected — the effect Eddington measured during the 1919 eclipse. A well-aligned foreground cluster smears background galaxies into arcs, occasionally into a complete Einstein ring, and magnifies them by factors of ten or more. That magnification is used deliberately: several of the most distant galaxies known were only detectable because a cluster in front of them acted as a lens. There is a weak version too, and it is the more useful one. Most background galaxies are only very slightly distorted, far too little to see individually, but averaged over millions of them the statistics map where the mass is — including the mass that emits nothing. Weak lensing surveys are currently the most direct way of weighing dark matter across the sky.