Here is a great video from the Perimeter Institute, Waterloo Ontario explaining the evidence for Dark Matter. In this video we learn how physicists calculate the mass of a galaxy. They calculate the mass using 2 different methods:
1) Light Method - where they calculate the mass of a galaxy by its overall brightness as compared to the brightness of individual stars.
2) Orbital Method -They can calculate the mass of a galaxy using the size, radius and velocities of the stars in orbit.
Calculating the mass of a galaxy should produce approximately the same value for both methods. As it turns out, when physicists use these 2 methods to calculate the mass of any galaxy, there is a big difference.
The light method assumes all the mass in the galaxy is producing light.
The orbital method does not rely on light, and basically calculates mass based on the movement of stars in orbit. This mass is far larger than the mass calculated using the light method. In the example given in the video, the difference between the methods is about 39 Billion suns!
This huge difference implies there is a huge mass present in the galaxies that does not produce light, does not reflect light or does not interact with any kind of light.
The way we know this matter is present is that we see its gravitational affects on other objects in the galaxy (star movement and mass).
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