The universe is a funny thing. It only weighs about a tenth of what is suppose to using current models and something interferes with the gravitational systems in many places. So the concept of dark matter and dark energy were born to help us comprehend just what the heck is going on out there. These two things play a big role in our space models aiding in warping galaxies and expanding the universe. However, since we know so little about them they are like the blank spaces on the maps of old. While there were no monsters in those; science fiction ingrained monster, space aliens, time travel, and more live in our imaginations of dark matter.
Astronomers were looking at a star system when they noticed some warping going on. For example one stated "A circular galaxy will become distorted to resemble the shape of a banana if its light passes near a dense region of dark matter." The group decided to map out the weirdness to see if there was any pattern. To their surprise dark matter was clumping in spots which mass was ten trillion (10,000,000,000,000) times the Earth's sun Sol! The matter seems to be there because several galaxies have collided here.