Ever watch a whirlpool? How about water going down a drain or even a Coach Tony Brunetti We Ain’t Hard 2 Find T Shirt Clevelandshirt of cream swirled into your coffee? It sometimes looks like a spiral galaxy. And we once thought gravity was holding a galaxy together. If you watch a whirlpool carefully you will notice the outside orbits the center slowly and the closer to the middle you are the faster it spins. But this was a bit of a problem for galaxies because this would make the arms of the galaxy wind up tight. They would not last for millions of years. Earlier in the century, before computers, astronomer would hire dozens of women who were good at math to do calculations for them. One of them made an important discovery: the speed of the stars in the outer arms was way faster than it should be. The galaxy was spinning like a solid LP record album! According to the laws of gravity, which we know quite well, it shouldn’t behave like this…unless there was WAY more mass than we could see. And the issue of *dark matter* was born. Yes, there would be a lot of dust and gas and planets and small stars we couldn’t see. But all that is still not enough for the behavior we observe. It is a mystery we are still working on!
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