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.

In addition, both clusters have significant mass and act like gravitational lenses. By imaging objects that are behind the clusters and understanding how the images are distorted by the intervening lens, we can map out the Bullet Cluster's mass. This is shown in blue.

Overlaying the two images shows that the mass is not where the matter is—hence, dark matter. This is only one of several collisions between clusters that show similar features—gravity without apparent matter—but the Bullet Cluster is, without doubt, the cleanest example of them all.

However, the Bullet Cluster shows something that is, arguably, more important: science works. Although the initial publication was touted as evidence for dark matter, it was quickly realized that the story may be more complicated than that. In fact, the story even started to shade toward the Bullet Cluster being evidence against dark matter. Theoretical physicists let their imaginations loose, bringing dark energy and modified theories of gravity to the table. But eventually, as the dust settled, thinking came back around to the original interpretation being correct.

Looking back at the Bullet Cluster today—how we got from here to there and back again—highlights how science works in that same clean manner. Data is king, but theory is the kingdom; you need both, and neither is set in stone.

Explaining the data raises questions

Shortly after the Bullet Cluster analysis was published back in 2006, scientists began to take a closer look at the data. Initially, it all seemed a bit puzzling. Attempts to model the collision didn't seem to work.

One of the cottage industries in astrophysics is modeling galaxies and clusters of galaxies. You can, in your computer, create two clusters that approximately match the mass distribution of some observations, then ram them together at any speed you like. You can also produce a model that has lots of different clusters and look at the statistics of the collisions to see what the average cluster crash looks like.

This two-step process tells us different things. One model tells us, given the observational data, how big the clusters were and how fast they were approaching each other when they collided. The second model tells us, given our Universe, what size of galaxy clusters we should expect and how fast they typically collide.

For the collisional model, it is not enough to match the distribution of visible matter and gravitational lensing that was observed. There are a whole raft of features that the models need to reproduce. As we mentioned above, the normal matter is so hot that it produces lots of X-rays. But it's not enough for a model to just produce X-rays; it should produce the same spectrum of X-rays—that is we should be able to predict the relative brightness of each color of X-ray. Other constraints have to do with the material in the clusters. During the collision, matter (ordinary matter, that is) is transferred between clusters. Our observations provide an estimate of how much is transferred, and the models should predict the transfer.

The second model is all about probabilities. When you map the results of the first model onto models of many galaxy clusters randomly colliding with each other, you should find that the predicted collision is not too extraordinary. Yes, it is possible that we hit the equivalent of a winning lotto ticket. But if the models predict that the cluster collision requires pretty exceptional conditions, we should probably assume that we've made a mistake somewhere. Or, more precisely, for every collision that requires extreme conditions, we should have observed lots that are within the normal range. Since we don't have lots of other collisions, the Bullet Cluster should be within that normal range

But the first papers published after the Bullet Cluster analysis showed that, maybe, just maybe, all is not well. Is the Bullet Cluster special?