Black holes are regions of space where the gravitational pull is so strong that nothing — not even light — can escape. Rather than empty space, black holes are chock full of matter that gets squeezed into a teensy space. 

Who discovered black holes?

Physicist Karl Schwarzschild accidentally discovered black holes in 1916, when he was figuring out a particular solution to Einstein’s general theory of relativity. He was trying to find the solution to the gravitational pull of a single, solitary, symmetric ball of matter — such as the sun at the center of our solar system. But that solution contained a peculiar feature: the theory behaved strangely at a specific radius, known today as the Schwarzschild radius.