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What exactly is dark matter?

That is the question that has been puzzling scientists for the past 8 decades.

In 1932, the Dutch astronomer Jan Hendrik Oort concluded, after measuring the motion of the stars in the Milky Way, that the mass of the galactic neighbourhood must be higher than the mass of the observed matter.

Around the same time, the Swiss astronomer Fritz Zwicky found the gravitational mass of galaxies, inferred from their rotation curve, to be approximately 400 times larger than the mass of their visible matter.

To give a general idea, the matter close to the center of the galaxy rotates almost like a rigid rotator, like a vinyl record. As we move further away from the center, the rotational velocity of the matter is dropping at a constant, proportional rate. So as we approach the outskirts of the galaxies, where the matter is less and less bound gravitationally, we would expect this velocity to drop to zero. However, observations differ from these expectations. It has been shown that the rotational velocity remains constant with the distance to the center, giving a constant, “flat” rotation curve. It appears as if there is some unseen matter keeping it together. This idea of some unseen matter is consistent with the previous conclusions that there is more to the universe than we observe.

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