During the 60's it was discovered that there was no speed difference between the stars that were in the center of the galaxy and those that were further away. However, Newton's laws said that the farthest stars of the galaxy orbit slower. To explain this phenomenon the term dark matter was created, which is invisible but has mass. While scientists are close to discovering how to detect dark matter particles, properties, such as their temperature, are unknown. It may be cold, or it may be hot.
