The universe is constantly expanding caused by a theory called the Big Bang Theory. Scientists believe in this theory because they can see the galaxies slowly drifting apart.
Black holes are also another theory widely accepted by scientists. Black holes the reminiscence of a star about 10 times the size of our sun, they form then the star collapses in on itself to form a area of infinite density.
The event horizon is associated with black holes. They are associated with black holes because it surrounds it and is completely isolated. The event horizon is separate from anything besides itself. The event horizon is also one of the strongest evidences of black holes existing.
Pulsars, pulsars are special neutron stars that spin rapidly giving off a strong radio waves. Pulsars are about the size of Manhattan, and they are like a lighthouse in the fact that you can only see the radio waves when its facing you.
Dark matter, dark matter is a type of matter that scientists theorize. Scientists believe this because there is a lot of unexplained gravity in galaxies. Some scientists do not believe in this because it is very hard to find because it gives off no electromagnetic waves or light.