Bullet Cluster
When two cars crash, the metal frames crumple and stop, but loose objects on the seats keep sliding forward and end up ahead of the wreck. Something like this happened when two galaxy clusters slammed through each other. The Bullet Cluster is the wreckage of such a cosmic collision, and it offers some of the most direct, hard-to-dodge evidence that dark matter is real and separate from ordinary matter.
Each cluster is made of three things: galaxies (which are tiny targets and mostly pass through untouched), vast clouds of hot gas (most of the ordinary matter, which collides, heats up, and slows down like a fluid), and dark matter (which, if real, should barely interact and sail right through). When astronomers mapped the Bullet Cluster, they used X-rays to find the hot gas — and it sat piled up in the middle, dragged to a halt by the collision. Then they mapped where the mass actually is, using gravitational lensing (the bending of background galaxies' light). The mass was not where the gas was: it had passed through and moved ahead, sitting with the galaxies on either side.
This separation is the punchline. Most of the cluster's mass is sitting apart from most of its ordinary (gas) matter — exactly as predicted if dark matter exists and slipped through the collision while the gas got stuck. It is hard to explain with modified gravity, because modified-gravity theories tie the extra pull to where the visible matter is, yet here the gravity clearly points to where the matter is not visible. The Bullet Cluster is often called a 'smoking gun' for dark matter, though it constrains rather than completely rules out every alternative.
Composite images of the Bullet Cluster overlay two maps: pink shows the hot X-ray gas trapped in the middle of the collision, and blue shows the total mass inferred from gravitational lensing, sitting in two clumps that have already passed through. The blue does not line up with the pink — the mass and the ordinary gas have come apart.
Mass (lensing) and ordinary gas (X-rays) sit apart — a strong sign of collisionless dark matter.
The Bullet Cluster is powerful but not a single magic proof. The fullest case for dark matter comes from combining it with galaxy rotation, the cosmic microwave background, and large-scale structure; each line of evidence supports the others.