Dark Matter Mystery
While carefully measuring the speed of rotation of galaxies, astronomers stumbled upon a profound cosmic mystery. Determining the gravity of the galaxy. They could estimate what the rotation speed should be by calculating the mass of all the visible stars and gas, thereby determining the gravity of the galaxy. Much to their surprise, the measurements showed that most galaxies are rotating faster than they should. Not a little faster. Much faster! More than twice as fast. This meant that, according to Einstein's theory of gravity, these galaxies should be flying apart. Yet clearly, they are not.
What can the answer be? Is it possible that most galaxies are surrounded by some 'dark' form of matter that cannot be observed by radio, infrared, optical, ultraviolet, X-ray, or gamma-ray telescopes? Could Einstein's theory of gravity, which has proved to be correct in all cases so far, be somehow wrong?
X-ray telescopes have discovered vast clouds of multimillion degree gas in clusters of galaxies. These hot gas clouds increase the mass of the cluster, but not enough to solve the mystery. In fact they provide an independent measurement of dark matter. The measurement shows that there must be at least four times as much dark matter as all the stars and gas we observe, or the hot gas would escape the cluster.
Chandra X-Ray Observatory Center