Two galaxy clusters collided like two swarms of bees flying through each other. One of them is the famous "Bullet." After the collision, the hot gas, which makes up most of the ordinary matter, slowed down and stayed in the center, while the galaxies, like small dense pebbles, flew on. Light from distant objects passing by is bent, as seen in distorted images.
Usually, this is explained as: since the bulk of the mass is in the gas, yet the light bending is stronger near the galaxies, there must be invisible dark matter there. But in the modified gravity theory MOND, attraction depends not only on mass but also on how densely matter is packed. Galaxies are tiny, superdense points compared to gas spread over millions of light-years. So they create strong attraction, even though their total mass is smaller.
The authors calculated what the light bending would be in MOND and compared it with observations. The result matched what general relativity with dark matter predicts. So the Bullet Cluster is not definitive proof of dark matter — both theories work equally well.
Cluster collisions are like a head-on car crash where the bumpers (gas) stay put, but the passengers (galaxies) fly forward. And the fact that the passengers catch the eye more doesn't mean there was invisible cargo in the car.
The Bullet Cluster got its name from the shock wave in the gas, which looks like a bullet piercing a cloud.
In the series "Dark Matter," characters travel on a ship that uses invisible mass for navigation, but as this work shows, the invisible isn't always necessary.