Science-in-progress: Did the Bullet Cluster withstand scrutiny?
Dark matter was first proposed to explain the speed at which stars orbit the center of their galaxies. Ever since, the search for other lines of evidence for dark matter has been an interesting one.
One of the biggest successes appeared to be a collision of galaxy clusters called the Bullet Cluster. It provided one of the most spectacular and intuitive indications that seemed to show that dark matter was real. Our own report on the first evidence of the Bullet Cluster, written more than a decade ago, was pretty excited. And in the stories that followed about the existence of dark matter, we've tended to treat the Bullet Cluster as a gold standard. If you can't explain the Bullet Cluster, then your theory is probably a bit useless really.
The image above shows the remnant of two galaxy clusters that have collided, with a smaller "bullet" that has passed through the larger cluster. The energy of the collision is such that regular matter has been heated to very high temperatures, causing it to glow like crazy in the X-ray regime (which is shown in red). So, an X-ray telescope can produce a clear image of the matter distribution of both the bullet and the larger cluster. Even better, this collision appears to be almost side-on to us, so we have the best seat in the house to observe it.