Black holes heavier than a trillion Suns act like cosmic umbrellas. They cast a shadow on the faint afterglow of the Big Bang — ancient light that fills the entire sky. The shadow turns out not blurred, but with a sharp edge, as if cut out with scissors: around it glows a bright ring of photons trapped by gravity.
But the real surprise is how the shadow behaves over vast distances. Due to the expansion of space, the apparent size of very distant objects stops shrinking. An umbrella carried almost to the very horizon of the cosmos suddenly starts casting a larger shadow than a nearby one. Those are the ones we need to look for — the farthest ones possible.
This effect lets us set a firm limit: black holes heavier than a hundred million billion Suns simply don't exist within the visible cosmos. Otherwise, their record-breaking shadows would have already been spotted. So these monsters aren't hiding in dark matter and don't play a leading role in the birth of galaxies.
If one of these black holes were at the center of the Milky Way, its edge (the event horizon) would lie beyond the orbit of Pluto.
In Interstellar, Gargantua was depicted, but real supermassive black holes are millions of times more massive.