00:00What is dark matter? Dark matter should be called invisible matter. It's matter
00:08that doesn't interact with light or with any other charged particles. It's matter
00:13that has nothing to do with light. That's the point. I'm Paul Sutter and this is
00:20Paul Explains, the show where I, you know, explain. How do we know that dark matter
00:27exists? Well, we don't see it in Earth or the solar system. You have to look at big
00:34scales before dark matter really starts to reveal itself. And we saw first in the
00:411930s with the motions of galaxies inside of galaxy clusters. The galaxies
00:47were just moving way too fast. The galaxy cluster should have ripped itself apart
00:52billions of years ago, but there was still existing. So something
00:57had to be gluing all those galaxies. Even our own galaxy, the Milky Way, just
01:02shouldn't be here. Then in the 1970s, we discovered that stars are again moving way
01:11too fast. There has to be an extra source of gravity to hold the stars in. And the
01:19source of gravity can't come from something we see like stars or nebula or
01:24anything else that glows. Otherwise we would have seen it. We would have
01:29accounted for it. There is something inside of galaxies. There is something
01:33inside of galaxy clusters that has mass, that has gravity, but isn't emitting any
01:39light. Since the 1970s, we've gone even further to solidify our understanding of
01:46dark matter or that we know it exists. From the earliest moments of the universe, we have
01:52the cosmic microwave background. This is leftover light from when the universe was
01:57just 380,000 years old. And by studying tiny little variations in that light, we can get
02:03a picture, a map of what the universe was like back then. And guess what? There was a lot
02:08of matter back then that didn't interact with light. So we have all these different pieces of evidence
02:14that all fit together and all point to dark matter. Very likely, dark matter or invisible
02:23matter is streaming through the room you're in right now. But it doesn't interact with light,
02:28doesn't interact with charged particles, it doesn't interact with normal matter. So you just don't see
02:33it. You just don't care about it. It's invisible to you. But at the very largest scales, it affects how
02:38things move. What we do know for sure is that the dark matter does exist.
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