Missing Mass Problem
One of the major unsolved problems of astronomy is the so-called ‘missing mass’ problem. First note that stars are not stationary. They swarm around inside their galaxy like flies around a pot of honey. By studying the way stars move inside galaxies today, astronomers are able to calculate the force acting upon them. And by counting the stars these galaxies contain, astronomers can calculate their mass.
Now the gravitational force of an object depends upon the mass it contains. It turns out that galaxies today have far too few stars, and hence far too little mass, to account for their gravitational force. If galaxies only contain the stars we can see and nothing else, they would not have a strong enough gravitational field to hold their stars together.
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