The first evidence of dark matter was galaxies that behaved as if they contained far more matter than could be explained by the mass of their stars. Since then, we’ve found many other indications that the universe is swimming with matter that we can only infer from the effect of gravity. The original evidence that galaxies of the dark matter structure persisted.

Until that awkward moment in 2018 when we seemed to discover the existence of Almost a galaxy devoid of dark matter. While there was doubt about this initial discovery, researchers eventually discovered a second galaxy that was very similar. This was a problem, since we had no idea how galaxies like this formed.

Now, researchers have announced that while running a model of galaxy formation, a number of dark matter-poor galaxies appeared naturally. And by tracing the model back to see how those galaxies arose, researchers can provide an explanation for these mysteries.

Join the dark side

Galaxies and dark matter are difficult to separate. Models of the early universe suggest that dark matter merged into filaments (which… we discovered) that meet in nodes that have a higher density of matter. These nodes, in turn, attracted the visible matter, causing clusters of galaxies to form around them. These galaxies will then merge a lot of dark matter, which has shaped their growth through mergers and structure processes to this day. Therefore, it is difficult to understand how a galaxy could end up with so little dark matter.

As a result, when the discovery of a seemingly dark matter-poor galaxy was first announced, doubts were high. Much of this uncertainty centers on the distance from the galaxy to us, which has been crucial to estimating its mass and thus its dark matter content. But the same team identified a second galaxy that appears to share many of the same characteristics as the first problematic.

Another reason to be suspicious is that we had no idea how this type of galaxy formed. We’ve done a lot of modeling of the formation of galaxies, including models that started before dark matter filaments formed. If dark matter-poor galaxies were a natural part of the universe’s evolution, we’d likely see models produce one. But there was no news of anyone attending.

So, we’re left with two eccentric apparent galaxies. We have questions about their actual characteristics and no hint of how they might look like they are. This is not a pathological condition.

updated model

But the solution may have come from different researchers trying to do something else entirely. What they were doing was modeling the evolution of galaxies using a state-of-the-art software package called FIRE-2 (for “Reactions in Realistic Environments”). Although they don’t say what they were trying to look at, they say they weren’t dark matter-poor galaxies: “We note that we didn’t expect this to happen beforehand (that is, our simulations weren’t originally designed for this purpose).”

Regardless of the intent, running the model produced seven galaxies that were nearly devoid of dark matter. The researchers named these names after the seven Cherokee clans, which means things like Blue, Deer, and Wolf but also things like Long Hair and Wild Potato. The last two sentences are set up for some unexpected sentences in the astronomy paper, such as “For example, the bird displays low-brightness S-shaped tails, while Long Hair doesn’t seem to bother them.”

Anyway, the useful thing here is that the model has saved information about intermediate states, which means we can run simulations in time and see how these imaginary galaxies ended up without dark matter. But what’s becoming clear is that those galaxies didn’t start out that way. All seven were, at some point in the simulation, like typical small galaxies with natural supplements of dark matter.

Instead, each small galaxy lost its dark matter through what you can see as a failed merger with a much larger galaxy. From the perspective of simulations, which rely on metrics such as the distribution of mass within a galaxy, the researchers define these as “close encounters.” But from the perspective of what a failed merger would look like if we imaged it while it was happening, most of the small galaxy’s stars would be within the region occupied by the galaxy’s larger stars. In short, the little galaxy looked like it was being swallowed up before it somehow appeared on the other side.

But the small galaxy did not appear unchanged. While up to half of its stars appeared, nearly all of the gas and dark matter ended up in the larger galaxy.

If the statistics shown in this model are generally applicable, there could be many more such dark matter-deficient galaxies outside the two galaxies we’ve found so far. The research team estimates that up to 30% of the massive galaxies in the universe must orbit nearby. Which sounds like an invitation to observing astronomers to get busy and find more.

Nature Astronomy, 2019. DOI: 10.1038 / s41550-021-01598-4 (About DOIs).