Astronomers had a fundamental problem in the year 2018: they found two very small galaxies that were missing dark matter.
I know that sounds weird, but it is as shocking to the astronomer as finding human bones next to dinosaur bones in a dig. They should not be there.
Dark matter is a huge part of the way we understand the Universe, and finding galaxies without them is not just weird, but contrary to the standard models used. The community was thrown into a tizzy by it.
Simulations of how galaxies grow and behave reproduce ones without dark matter, and in fact they look an awful lot like the ones actually seen, are the first big step to the answer. It is not clear why the simulations reproduce them, but the galaxies in the sim have some characteristics that lead to a way to figure that out.
We call it dark matter because it dominates the Universe but is completely invisible. The kind of matter we make out of protons, electrons, and so on have mass, interact with each other, and also interact with light, making them visible.
In the 1960s and 70s astronomer Vera Rubin showed that there was a dark matter that was 5 to 6 times the mass of the Universe, and that it was invisible. What we call normal matter is in the minority.
It does not interact with normal matter either. If you put a big cloud of normal matter gas in space, it will collapse, but it will also feel pressure as the particles collide, and heat up as well. Dark matter will ignore the normal matter and only use gravity to do its thing.
Dark matter played a bigger role in how we understood the Universe after we realized it existed. It is thought that it was the first material to collapse after the Big bang, forming a huge swath of light years long. The dark matter framework like scaffolding allowed normal matter to 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609-
The dark matter attracted the dark matter to the galaxies. The dark matter haloes that surround all the galaxies are detected through their influence on the stars, gas, and dust.
Most small galaxies have an overabundance of dark matter. Massive stars blow fierce winds of gas, and exploding stars blast huge amounts of materials into space. These small galaxies are lost to space because they don't have enough gravity to hold on to normal matter. Dark matter blows past normal matter through things like gas pressure. The ratio of dark to normal matter is very high in the end, because the galaxy still has its initial dark matter.
No one expected to find exceptions. But.
Two extremely faint galaxies were found in the sky around the big galaxy in 2000. These are small, faint galaxies that are barely visible. It took some time to figure this out, with some really fun twists and turns along the way, but they discovered that the total mass of the two galaxies were 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- They were made completely of stars, with no dark matter at all.
The standard model tells us that is not possible.
One idea was that a special kind of dark matter could solve the problem.
We have a lot of knowledge about the physics of gravity, gas flow, star formation, and other things. This physics can be written as a series of equations that interact with each other, and those equations can be plugged into a computer to model how galaxies are born and behave over time. The physics is complicated, but a relatively new code called Feedback In Realistic Environments has been very successful in creating simulations that look and behave like real galaxies.
A team of astronomer looked at the code. Seven small galaxies in their simulations appeared to have lots of matter in the form of stars, but very little to none in the form of dark matter. The lead author of the work notes that he has indigenous ancestors and ends his writing about colonization and assimilation, and is worth it.
They discovered that all of them were inside the haloes of dark matter of nearby large galaxies, and that they had all recently passed through them. All seven of them had passed within 5% of the dark matter haloes from the center.
These interactions are keeping the normal matter intact while stripping away the dark matter. It is not clear why at the moment. The results are important, but they are still working on why. The hypothesis is that stars tend to have a circular path in these galaxies, but dark matter doesn't feel any pressure to do so. It is farther out from the center and more easily stripped by the bigger tides.
The curious case of the dark-matterless small galaxies will be solved if this is the case. The simulations show that at least one small satellite galaxy without dark matter should be present in 30% of all massive galaxies. It is a prediction that can be tested observationally. It is hard to measure the mass of those galaxies. We may have more data soon to see if it supports or refutes this hypothesis.
The AGC 114905 appears to lack dark matter, but is also isolated in space.
We have to say a lot when it comes to cutting edge science. There will be more news on its way.
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