Imagine the Earth shrunk down to the size of a marble. That's how dense a black hole can be!Imagine throwing a ball really, really hard. Even the fastest throw wouldn't be strong enough to overcome a black hole's gravity. Not even light, which is super fast, can escape!Scientists are still learning about black holes, but they think they form when very massive stars run out of fuel and collapse in on themselves.Black holes themselves are invisible, but astronomers can learn about them by observing the way they affect things around them. For example, they can sometimes see swirling clouds of gas and dust getting sucked into a black hole at super speeds.