A black hole is like a dark ball in space onto which luminous matter falls. A black hole has a shadow — a region from which rays cannot escape. But due to light bending, the shadow appears larger than the hole itself: just like a ball's shadow on a wall grows bigger when the lamp is far away.
It is proven that for a symmetric hairy black hole, the shadow radius is always at least 2.6 times the radius of the point of no return ($r_{\text{sh}}/r_{\text{H}} \ge 3\sqrt{3}/2$). The simplest 'bald' model gives this limit; any 'hair' enlarges the shadow. Remarkably, the shadow is not the hole itself but an illusion caused by the bent light of surrounding matter. Observations by telescopes like the Event Horizon Telescope turn shadows into a test for exotic physics. The shadow of M87 matches a 'bald' black hole, supporting Einstein's theory.
The shadow of the M87 black hole, imaged in 2019, matched the predictions of general relativity: circular and with a radius 2.6 times larger than the point of no return.
The film 'Interstellar' used actual light-bending calculations to create the shadow of the Gargantua black hole.