What crucial insight about the nature of dark matter was provided by observing the Bullet Cluster collision?

Answer

Dark matter passed right through the collision site largely unaffected.

In the Bullet Cluster, the ordinary matter (gas) smashed together and slowed down, while the dark matter passed through without slowing, demonstrating that dark matter is collisionless and interacts only weakly, if at all, beyond gravity.

What crucial insight about the nature of dark matter was provided by observing the Bullet Cluster collision?

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