I've always wondered about black holes. In the area of one, gravity prevents anything -even light- from escaping it.
How does something like this exist? Well, when gigantic stars collapse at the end of their 'life' black holes can form. Because the object is so dense, it sort of bends space and it's gravity attracts things close to it. The closer to a black hole, the more space-time is deformed. Theoretically, if someone could survive going to a black hole, while they get closer and time seems normal to them, anybody that could observe this happening would see the person slowing down considerably, possibly even looking as if they aren't moving (if you could see them at all.) Black holes are able to grow larger by taking in many stars and maybe even other black holes. When this happens, they are called supermassive black holes. There are 4 sizes of black holes. From smallest to largest: micro black hole, stellar black hole, intermediate-mass black hole, and supermassive black hole.
White holes, on the other hand, are only hypothetical. They are the opposite of black holes. They wouldn't be able to be entered from outside of it. However, light and matter could come out of it. This would essentially be like a worm hole if on the other side there was a black hole or entrance of some sort.