The Bullet Cluster observation, where colliding galaxy clusters show gravitational distortion concentrated with the galaxies rather than the interacting gas clouds, is strong evidence that dark matter is real rather than a failure of our understanding of gravity.

151:23more likely. One of them is called the Bullet Cluster. So the Bullet Cluster, there are two large clusters of galaxies. In these large clusters of galaxies, well, any galaxy consists of a couple of components. There are the galaxies themselves, there is the hydrogen gas that surrounds the galaxies, and maybe there is dark matter. And if dark matter is real or dark matter is not real, you will get different answers if those two galaxies pass through one another.

151:54The galaxies themselves should pass through one another basically not interacting, but the big thing is the gas clouds. So if there's big clouds of gas, as the galaxies pass through one another, the clouds should interact, and the gas clouds should stop in the middle and be really, really hot. So then you would see, if there were no dark matter, you would see a cluster of galaxies, cluster of galaxies, a big gas cloud in the middle,

152:18and because the big gas cloud in the middle is much more massive than the galaxies themselves, you would expect to see distortions that we call dark matter distortions in the middle. If, however, dark matter is real, the galaxies pass through one another, the cloud stops, dark matter doesn't interact with the cloud, so it passes through. In that case, you would expect to see the distortions where the galaxies are.

152:43And that's what we see. So that is strong evidence in my mind. The Bullet Cluster is strong evidence that dark matter is a real thing. And there is another example which is much more recent, dark Bullet Cluster was a while ago, called the Dragonfly galaxies. There's Dragonfly 2 and Dragonfly 4. These are galaxies that rotate exactly according to Newton's laws. And so the fact that they rotate exactly according to Newton's laws

153:13says that whatever's causing galaxies to rotate too fast is not a property of matter. But if you had a galaxy where there was no dark matter, for whatever reason it got stripped off or something, this is one of those lovely ironies that the existence of a galaxy with no dark matter is very strong evidence that dark matter is real because you can take the dark matter out. So the DF2 and DF4 also suggests to me that dark matter is real. So now,