Are there any dark stars or dark galaxies made of dark matter?
I understand that 90 to 95 percent of the mass in the universe is dark matter, and this dark matter interacts only with gravity. If this is true, is it possible that there are dark worlds orbiting dark stars? Perhaps even dark galaxies?
You are absolutely right in that 90 to 95 percent of the mass in the universe is dark matter. However, there are not likely to be dark stars.
Current models of dark matter depict them to be some non-baryonic matter (baryonic matter means ordinary matter) which are very weakly interacting so that we do not feel its presence other then through its gravitational effect. In the formation of galaxies, it is the dark matter that collapses first and provides the gravitational force for ordinary baryonic matter to collapse. However, in order for matter to collapse enough to form stars, it must lose a great deal of angular momentum. Ordinary matter can lose angular momentum through collisions between the particles, but dark matter particles don't do this.
As a result, there are no such things called dark stars. Further, the galaxies that are visible to us mark the places where dark matter is concentrated; the matter that we see is located in the high density regions that are traced out by dark matter.
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