The dark side of the universe
Das Kernelement des CRESST Experiments detektiert sogar Teilchen, die mit Materie kaum interagieren | © Max-Planck-Institut für Physik

Five times more dark matter than normal matter in the universe

There are many indications for the existence of dark matter. The core of all observations supporting the theory of dark matter is that there is far more mass and consequently more gravitational force of attraction than expected by counting all stars, galaxies and other matter. It can be learned, for example from cosmic microwave background and the distribution of galaxies in the universe, that the amount of normal matter would not be sufficient to build the existing large scale structures. Current measurements indicate that only approximately 15% of all matter in the universe is made up from particles already known.

Evidence on much smaller scale can be found by having a look at the distribution of the stellar rotational velocities in galaxy. Analogous to our own solar system where the planets spin around the sun, inside of a galaxy stars fly around a common galactic center. However, the time and length scales are a good deal larger than the ones in planetary systems. Our solar system, for example, takes 200 million years for one orbit around the Milky Way.

Of course, the velocity of an object depends on the distance to the gravitational center. The closer the object is to a huge mass, the faster it has to move to assure a stable orbit. The earth travels at a speed of more than 100.000 km/h around the sun. A bit less and it would slowly spiral into the sun, a bit more and it would drift away eventually leaving the planetary system. For comparison: Mercury, the planet closest to the sun, moves with more than 170.000 km/h. The same argument is valid for stars in galaxies. Stars farther away from the galactic center should move at a slower pace. But measurements of the velocities show, that beginning at a certain distance to the center, all stars have about the same velocity — no matter how far outside. Nevertheless, they are bound to our galaxy.

The explanation believed to be correct by the vast majority of physicists is the existence of dark matter. Then the velocities of the stars could be explained by additional dark matter distributed on large scale across the galaxies. Consequently, there would be more matter between the center of the galaxy and the stars farther outside than between the center and the stars close to it. Stars at the border of the galaxy would feel a bigger force due to the additional attractive mass and would not drift away, even at high velocities. In this case, all observations can be understood. But: what would this dark matter be?