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Dark matter, dark energy, there are all kinds of things in space that we have yet to figure out. But your real hipster astronomers don’t worry about famous mysteries like that.

They worry about stuff like the missing baryon problem: the fact that around a third of the regular, ordinary matter in the universe has refused to show up on any telescopes. At least, until now. In last week’s issue of the journal Nature, an international team of astronomers reported that they’ve finally found evidence of the missing matter.

Honestly, it was right where everyone expected it. But it took decades of new techniques, new telescopes, and new knowledge to finally find it. We have lots of ways of figuring out how much stuff is in the universe.

For example, we can look at the Cosmic Microwave Background, the echo of the Big Bang whose colorful pattern depends on the universe’s early makeup. Or we can see how quickly the universe expands, which depends partially on the gravitational pull of all of its matter on all of the other matter. We can also look at how quickly early structures formed.

The list goes on from there, but one of the greatest achievements of modern astronomy is that all our independent measurements using different techniques tell pretty much the same story. Most of the universe is dark energy, a poorly-understood, nonstop pressure spreading apart space. Then, most of the rest is dark matter, a kind of something that exerts a lot of gravity but doesn’t emit or absorb any light.

Finally, a measly 5% of the universe is ordinary matter, or baryonic matter, it’s the kind that we and the Earth and the stars are made of. Everything we’ve ever seen or touched, all the countless stars and planets, all of that is just 5% of what’s out there, with the rest being stuff that we fundamentally don’t understand. Except, there’s also a problem:.

We can’t actually find a big chunk of that ordinary matter. When we count up the matter in all the stars, galaxies, and gas clouds we can see and then extrapolate, we find that ordinary matter makes up only about 3% or so of the universe. That means around a third of it is missing from our observations: We know it’s there from all those other measurements, but we haven’t seen it with our telescopes.

And this is just normal matter! This is the stuff that should be relatively easy to find! Astronomers have dubbed this “the missing baryon problem”, where baryonic matter is stuff made of protons, neutrons, and electrons.

Most physicists have a slightly different definition of baryon, but this is the one many astronomers use. So far, researchers have mostly assumed that the missing baryons are hidden in certain immense filaments of gas that sit between galaxies, generally known as the Intergalactic Medium, or IGM. The IGM can be millions of degrees Celsius, but the gas in it is so sparse that it would feel cold if you were in the middle of it.

Astronomers think that a huge fraction of the universe’s mass is tied up in these filaments, but the IGM is hard to investigate directly. See, it’s mostly ionized hydrogen, or hydrogen that has lost its one and only electron. But atoms give off and absorb light when the arrangement of their electrons changes.

So if the hydrogen in the IGM has no electrons, it won’t absorb or emit any light. Which makes it really difficult to study. Thankfully, research over the last few decades has also indicated that there should be a tiny amount of ionized oxygen in the gas, too.

Oxygen starts out with more electrons than hydrogen, so when it loses some and becomes ionized, it still has others left over. So we can try to see it. And from there, we can extrapolate how much other stuff is in those filaments of the IGM.

To find evidence of that miniscule amount of oxygen, the astronomers in this new paper used the European Space Agency’s XMM-Newton, an orbiting X-ray telescope that can see the kind of light that interacts with ionized oxygen. But there’s so little oxygen in the IGM that it doesn’t just shine like a star; you need a huge flashlight lighting it up. For this team, that flashlight was a distant type of quasar sitting conveniently far behind the gas.

These are objects powered by black holes that emit tons of radiation, more than many galaxies. They looked at the quasar for about 18 days total over the course of two years, and all their observations told them that some of the its light was getting absorbed by the. IGM’s oxygen on its way to Earth.

Based on how much light was absorbed, and how much oxygen we think is in the IGM compared with other elements, the team concluded that there’s exactly enough matter in the IGM to account for the missing baryons. Even with two years of observations, though, this isn’t the end of the mystery. The team admits that there’s still a lot of uncertainty, and that we need to look at more filaments of IGM before the problem will truly be solved, just in case this gas is an anomaly.

But this paper does show that there’s a light at the end of the tunnel and that our hypotheses are on the right track. And once the missing baryon problem is solved, we’ll just need to figure out the other 95% of the universe, and we’ll be all set! Thanks for watching this episode of SciShow Space!

If you would like to keep learning more about the universe and how weird and cool it is, and stay up-to-date with astronomers’ newest findings, we’ve got you covered. You can go to youtube.com/scishowspace for more videos and to subscribe. [♪ OUTRO].