Black hole
Since we cannot see black holes, they must be detected by other means. When a black hole passes between us and a source of light, the light bends around the black hole creating a mirror image. That effect is called gravitational lensing.Dyson F.W; Eddington A.S. & Davidson C. 1920. A determination of the deflection of light by the Sun's gravitational field, from observations made at the total eclipse of 29 May 1919. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 220 (571–581): 291–333.