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When you picture a galaxy, you’re gonna be imagining stars and planets; maybe some clouds of dust and gas. But the truth is, most of the galaxy is invisible, held together by something called dark matter.

It is an invisible “stuff” about five times more abundant than regular matter, but we can only observe it by watching its gravity affect other objects. Most astronomers agree it exists, but there are still a lot of questions, to the point where some researchers hypothesize dark matter may not be real at all. In the past, these alternative ideas have had fatal flaws.

But two weeks ago, two researchers proposed a new model in the journal Physical Review Letters that is the first to pass one crucial test. So dark matter was proposed in the 1930s, and today, there’s a good amount of indirect evidence for it. Usually, it pops up when something in space seems to be missing mass.

Normally, how massive something is is directly linked to how strong its gravity is. More mass, equals more gravity. Except, astronomers have found clusters of galaxies with way stronger gravity than their mass suggests, like, ones whose gravity distorts and magnifies light more than they should.

That “missing mass” is dark matter. But because no one has collected direct evidence of it, it’s possible it might not exist. Instead, there’s another idea that maybe gravity doesn’t work the same way on every scale.

Maybe it’s different in a solar system compared to a huge galaxy cluster. The trouble is, models trying to mess with gravity haven’t been able to match everything we see. Like, take the cosmic microwave background, or CMB.

It’s basically our universe’s baby picture. It’s a snapshot of light only 400 thousand years after the Big Bang. And it has a very particular pattern, with tiny temperature variations across the universe.

Models without dark matter haven’t been able to recreate that. At least, until now. In this new paper, two scientists appear to have done it:.

They have developed a model that can replicate the CMB without dark matter. Like some attempts before them, they introduce two new fields to the universe that act as an extra source of gravity. But what’s different is how those fields evolve.

In this model, the fields act one way at the start of the universe; they mimic what we call dark matter in such a way that they recreate the CMB. But then, those fields evolve into a force that was described in that older work, a force that makes gravity overall act differently on really large scales. Now, a key thing is, this wasn’t based on any fundamental theory:.

The team was just tweaking a model to match observations. But the fact that it’s the first to replicate the CMB without dark matter is an achievement. I mean, it’s not the end of the road:.

This idea also has to be tested against other observations. Like, it will also need to explain what we see in those galaxy clusters. But this is what science is all about: questioning the status quo, testing hypotheses, and refining ideas about the universe.

And sometimes, like in this next story, it’s about refining ideas about ourselves. Last week in the Journal of the American Heart Association, researchers reported another way to detect the effects of space on our bodies: watch for DNA leaking out of our cells. So, space travel is rough.

Not only does the microgravity make your face all puffy as fluids get redistributed, but your muscles and bones start to break down because you don’t need them as much, and your immune system weakens. So, scientists are always looking for ways to monitor astronauts’ health and keep them safe in the long term. In last week’s paper, one team turned to mtDNA.

This is the DNA inside your mitochondria... AKA the powerhouse of the cell, because they make usable energy for your body. When mitochondrial DNA gets damaged, some of it can end up outside the cell, and previous research has suggested that this “cell-free” mtDNA can play a role in diseases.

So, in this paper, the team wanted to see how much mtDNA leaks out of astronauts’ cells during a short mission. They studied 20-year-old blood samples taken from 14 astronauts prior to their trip, as well as the samples taken the day they got back to Earth, and those from three days after that. And they found that across all the astronauts, the amount of cell-free mtDNA increased after returning to Earth.

But it also continued to increase in the next three days. Since they didn’t have samples from beyond that, they don’t know how long the effect lasted. But they did find that it varied massively.

Like, one astronaut saw their cell-free mtDNA double, but another’s was 355 times higher than what it was before they left Earth. This suggests that things like an astronaut’s genetics and general health play some kind of role in this. But the samples were anonymised, so the team couldn’t look into that.

This study also had a small sample size, even compared to the already small number of astronauts in the world. And it only looked at people going on missions less than two weeks long. But overall, this research suggests that measuring the amount of mtDNA leaking out of an astronaut’s cells could hint toward other health problems that may be down the line.

Here on Earth, increased amounts of cell-free mtDNA have been shown to predict the onset of neurodegenerative diseases, cardiovascular diseases, diabetes, and rheumatoid arthritis. So using blood tests to watch for these increases in space could possibly get an astronaut treatment earlier than they would get otherwise. Again, there’s a lot more questions scientists need to answer.

But along the way, they will be learning more about the human body, and how to keep people safer on their quest to understand the universe. Thanks for watching this episode of SciShow Space! If you would like to help support this channel, we have a Patreon at Patreon.com/SciShowSpace.

You can go there right now and learn more about how you can help us keep making good, cool space content. [♪ OUTRO].