In: Physics
an Astronomy question, please answer it comprehensively. Thank you
List (and explain) two reasons astronomers suspect that Dark Matter exists.
There is so much evidence on the existence of dark matter. This is not related to any science fiction but it is just a term for missing mass which we are not able to see.
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Galaxy cluster -
we know that almost all celestial objects move around, about each other and they are gravitationally bound. When we calculate the gravitational mass of the cluster using its kinetic energy and its total potential energy and then compare it with luminous mass ( visible through light) then we often find that they are not same. There is a considerable difference between gravitational mass and luminous mass. What does this mean ??
This means that there is something hidden from our eyes, something dark which is holding the cluster together. This is often called as missing mass aka dark matter.
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Galactic rotation curves
From our common understanding, we know that velocity decreases as we move away from the center of galaxy. A peculiar and interesting observation was made in Andromeda galaxy where all the stars, had same constant velocity irrespective of the location and Andromeda is a spiral galaxy . This means that there is dark matter in between the arms of galaxy and a large fraction of the galaxy’s mass resided in a diffuse dark matter halo which is beyond what we observed through light.