Black holes are certainly the most unknown ‘things’ in the universe. Despite this we still do know somethings like, at the centre of the milky way is a supermassive black hole and that they have very strong gravity.
How they are created
In a star, radiation pushes agains gravity, when there is fusion in the core this stays stable. However in stars that are big enough and powerful enough to fuse iron, eventually loose this balance because the fusion process that makes Iron doesn’t release energy. In a fracation of a second it emplodes and it is in this moment all the denser elements in the universe are made. (quarter of the speed of light). This causes a supernova and then creates a neutron star but if the star is big enough all of its mass collapses into a black hole.
Black holes come in all shapes and sizes, from the size of an asteroid or 40 billion times the mass of our sun.
When you look at a black hole you are actually looking at the event horizon, the actual black hole is the singularity which is possibly an infinitely dense object with nearly an infinite gravitational force, when you get to a certain distance away you would need to go faster than the speed of light to escape, which is impossible. Inside the event horizon the gravity is so great that it actually bends spacetime itself, so anything inside will experience slowed time and see the universe in fast forward.
What happens if you go in a black hole?
What happens if you go into a black hole? Well the chances are because of the extreme gravity, your atoms would be torn apart from the closest point to the singularity to the furthest point from it. Or it is possible that soon after you cross the event horizon you hit a fire wall and are immediately killed.
Do black holes ever die?
All black holes eventually evaporate through a process called Hawking radiation. Empty space is full of ‘imaginary particles’ that collide with their anti-particles and then disappear. When this happens on the edge of a black hole one of the pairs is sucked in and the other escapes meaning the black hole loses some energy, however this process takes millions of years.