How was it discovered?
It is also known as invisible matter. It is called this because it doesn't give off, reflect, or absorb light. The reason we know it is actually there is because it has gravity. It is what most of the universe is. 85% of our universe is made of dark matter. The first person to argue about the existence of dark matter was Fritz Zwicky. He was studying the coma cluster, and something jumped out at him. Odd star patterns made him believe something was there holding them together. That is how dark matter was discovered.
The light blue in this picture shows how the dark matter holds matter together with gravity.
The red in this picture is dark matter and the blue is its gravity.
Dark matter can be spread out too.
What is it?
Scientists have wondered what dark matter could be. They thought it could be MACHO's, or massive and compact halo objects. Soon though, scientists decided that they couldn't work. The reason they didn't work out is because there are way too few of MACHO's to account for all of dark matter. Some thought they could be WIMP's, or weakly interacting massive particle. They ruled out WIMPS's too. WIMP's have a weak nuclear force, and some of the places with dark matter didn't have that force. Most scientists today believe that dark matter is some undiscovered subatomic particle.
"Dark Matter." Chicago, Illinois: World Book, 2013. Print.