If you wanted to go to space, how would you do it? Probably you’d build a rocket or a space shuttle and burn through tonnes of rocket fuel to leave the Earth’s surface, right?
It takes so much rocket fuel because to go to space you need to overcome the Earth’s gravitational field. The heavier a planet is, the stronger its gravitational field is and the faster you’d have to go to get out of that field. So on Earth you need enough fuel to go at 25,000 miles per hour. If you were on a bigger planet you’d need to go even faster. The faster you want to go, the more fuel you need to burn.
But what if you wanted to escape the gravity of something even bigger? Say, a star? Well, for a star it would be difficult, but it would be possible. However, there are objects in the Universe that are so massive that you could never go fast enough to escape their gravity. We call these objects black holes.
Black holes are what happen when giant stars run out of fuel at the end of their lifetime. After enormous, bright “supernova” explosions, all that’s left over is a super-dense, super-heavy core, called a black hole. The gravitational field around a black hole is so strong that you’d have to be travelling faster than the speed of light to escape it.
Nothing can go faster than light, so not even light itself can escape the gravitational field. What happens when no light can escape something? It looks dark. Hence, a "black" hole. We can still detect them, though, because we can see the gravitational effects they have on stars around them. We even think that there’s a black hole at the centre of our own galaxy.