The modern understanding of black holes started with the physicist and mathematician Albert Einstein when he published his theory of gravity in 1917. Since then, scientists have used Einstein’s equations to develop a more precise picture of these massive and dense entities in space, so dense that not even light can escape their pull.

According to physicists, anything can be theoretically turned into a black hole. For instance, when compressed into a smaller size, rock continues to collapse until it disappears from sight. However, its disappearance does not mean it has disintegrated. Einstein’s theory states that the rock’s mass and gravity survived. This remaining gravity now disturbs the space and time surrounding it, and this disturbance of time and space is the black hole.

In the natural world, the only things that can turn into black holes are very large stars, those that are many times larger than our Sun. When these massive stars reach the end-point of their lives, they collapse upon themselves. With their gravity and density surviving the collapse, a black hole is formed. While huge stars measure millions of miles across, the black hole they turn into is only a few miles wide and shaped like a sphere.

The black hole is the densest place in the universe. Even if a light is shined on it, no light will be reflected because the light is engulfed by the black hole. The rim of the hole has been called by physicists the “event horizon.” Anything, not only light that floats into the hole’s rim, will be engulfed and will never be seen again. This is why black holes are viewed as the exit points of our universe.

However, black holes do not only engulf light and radiation. As strange as it may sound, the lapse of time slows down as one comes nearer a black hole, while the lapse of time farther from the hole remains relatively faster. This can be found in Einstein’s theory, which holds that anybody that is massive enough slows down the flow of time. This is called gravitational time dilation, where the gravity around a massive object forces time and space around it to curve. And so, the stronger the gravity, the slower time passes by. This has been confirmed here on Earth by scientific instruments, even though our planet’s gravity is weak and the slowing down of time is barely noticeable. At sea level, you grow old one-billionth of a second slower every year than you would if you stayed at the summit of Mount Everest. At the top, you are farther away from Earth’s mass and gravitational pull. Compare Earth’s gravitational pull with a black hole’s, and it is not hard to imagine how bizarre time behaves on a horizon. An external observer, for instance, will see that an object nearing the black hole will seem stuck in time.

Einstein’s theory of gravity anticipates that in the interior of the black hole, time itself stops and gets destroyed. Some physicists have called this reverse creation. Einstein’s theories seem to hold true inside a black hole, but a breakthrough regarding details of the interior of a black hole may require new major advances in physics.