Black holes are objects in space-time where gravity is so enormous that nothing, not even light, can escape it. Here are five facts about black holes that you probably don’t know.

1. Black holes are the most luminous objects in the universe, that they produce no light. Instead, when black holes collide, they twist space-time and cast-off energy greater than the luminosity of all the stars and the universe combined, but in the form of gravitational waves.
2. Inside a black hole, time exists. However, time moves toward the black hole’s singularity, which is in the black hole’s center. That is why nothing can escape from a black hole, because that would require moving backwards in time and nothing can do that.
3. A stellar black hole formed from a degenerate massive star may hold the mass of 30 or more suns, but spans only 30 kilometers across, which is about the size of a city.
4. Every normal galaxy has a supermassive black hole in its center. Some are active, shooting jets out of their galaxy’s centers, while most are quiet, hibernating, waiting for more material to move close to feed on. If that happens, black holes in the centers of galaxies reawaken in a spectacular blaze of energy.
5. Near a black hole’s event horizon, also known as its edge, time slows down to a near standstill. If you could move into a closed orbit near a black hole and stay there for a while and then return home, you would discover that millions of years had passed on Earth. Even though you only seem just a short-time older, all of human history would have faded into a distant memory.