The gravity of a black hole creates a curvature of spacetime, acting like a lens: light from a distant galaxy splits in two, stretching into arcs. Dark matter around the hole barely changes the picture, as if a transparent curtain was added to a window pane. But light from the different images arrives with a delay — from minutes to years. And it's this interval that slightly shifts due to the invisible substance.
Calculations show: even for giant holes, the image distortion is less than a thousandth of a percent. However, the arrival time difference of the 'twins' shifts by fractions of a second — atomic clocks can notice such a thing.
Time becomes the key to the invisible halo. And although image distortions are negligible, future telescopes will catch microsecond shifts. Paradox: the delay from the hole itself can be so gigantic that the light set off before the first humans appeared on Earth.
In 1912, Albert Einstein predicted that gravity bends light; the first mirage ring was found only in 1979 with the help of radio telescopes.
In 'Interstellar', the heroes study the black hole Gargantua by the distorted light of stars — the scene almost exactly mirrors the calculations for the real M87*.