gravitational lensing

Gravitational lensing is the bending of light by mass that lies between us and a distant source. Because mass curves spacetime, and light always follows the straightest available path through that curved geometry, a ray passing near a galaxy or cluster gets deflected, like a straw that looks bent where it enters water. A massive object thus acts as a giant, lumpy lens, redirecting light that would otherwise have missed us into our telescopes.

When the alignment is close to perfect, a background galaxy can be smeared into glowing arcs, split into multiple images, or even bent into a complete circle of light called an Einstein ring. When a smaller object drifts in front of a star, it can briefly brighten the star without distorting it, an effect called microlensing that has been used to detect distant planets. The same mass also magnifies faint, far-off galaxies, letting a natural lens act as a free cosmic telescope.

Lensing has become one of astronomy's sharpest tools because it responds to all mass, whether or not that mass gives off light. By measuring how strongly the background is distorted, astronomers can weigh galaxy clusters and map where their matter sits, and this is exactly how we find that most of the mass is invisible dark matter, often spread far from the visible stars and gas.