[ intro ].

Here's a weird thing:. Based on our understanding of physics, the universe as we know it… should not exist.

In fact, all matter — the stuff that makes up you and me and basically everything you see around you — should have been destroyed not long after it formed. As for why it wasn't… well, that's actually one of the biggest unsolved mysteries in science. It's something we're very slowly starting to unravel, and every year, scientists are uncovering new leads that could help us figure out how and why we're all even here.

Here's what we know so far. The reason we're so confident we shouldn't be here is based on our understanding of antimatter. When you hear that word, it might conjure up visions of this hypothetical, mysterious substance, but that's because the name like stinks.

Antimatter definitely isn't hypothetical. And despite what it's called, it isn't the opposite of matter, either. In reality, antimatter is just like normal matter, but it has the opposite electric charge.

For instance, if an electron has charge of minus one, then an anti-electron — which is called a positron — has charge of plus one. In every other way, they're identical. We can study antimatter in radioactive decays and particle accelerators, so we've been able to learn a decent amount about it.

And as far as we can tell, every kind of matter particle has an antimatter counterpart. But when most people think of this stuff, they're not thinking about cataloging particles and misleading names. They're thinking about explosions.

If there's one thing antimatter is known for, it's that when a particle of it touches its exact matter counterpart — like, say, an electron hitting a positron — it all goes boom. It happens in a process called annihilation, and when it's over, the original matter and antimatter particles are gone. Usually, the only things left behind are some photons of extremely energetic gamma-ray light.

And that's where the problems start to come in. See, as far as we can tell, the Big Bang should have created equal amounts of matter and antimatter. It was all bunched up close together in a sort of “soup” made of hot, dense plasma, and these particles were running into each other constantly.

Annihilation was happening all the time. Now, to be fair, matter and antimatter were also forming during these early days. It happened during something called pair production.

It's the reverse of annihilation, and it happens when a photon with enough energy turns into two particles: one made of matter, and one made of antimatter. Pair production occasionally happens now, but it's nowhere near as common as it was billions of years ago. During the beginning of the universe.

Still, because it always produces equal amounts of matter and antimatter, it shouldn't have upset that fifty/fifty balance. So when the universe cooled and expanded, and pair production dramatically slowed, that ratio should have been locked in. Annihilation should have continued to rage until there was nothing left but photons.

But obviously that is not what happened, which is great. Instead, we have good reason to believe that essentially all of the particles left over from the beginning of the universe are made of matter, and that almost none of them are antimatter. And that just doesn't make sense.

It's not like there are secretly huge pockets of antimatter out there, either. If there were, there would be a constant stream of annihilation reactions wherever those pockets touched matter. And we'd be able to see a huge number of gamma rays as a result.

The only antimatter we actually see is the occasional stray particle from space, the short-lived particles made in radioactive decays on Earth, and the handful of particles made in accelerators. Technically, it isn't a hundred percent impossible, but it would be really hard for big chunks of antimatter to fit in with our current evidence for the structure and evolution of the universe. So the more likely explanation is that the modern universe contains essentially no antimatter.

But… why? The best explanation we have right now is that, for some reason, more matter survived those early days of chaos. Evidence suggests that for every billion antimatter particles made in the early universe, about a billion and one matter particles were made.

So for every billion annihilations that made photons, one extra matter particle survived. That's just bizarre, though. Based on what we've seen, matter and antimatter should be identical.

They should form the same way, and should decay into other particles at the same rates. They should also be treated the same by what we consider the three fundamental forces in particle physics: the strong nuclear force that holds together atoms, the weak nuclear force that governs atomic decay, and electromagnetism. So basically, if you imagine swapping every matter particle with an antimatter one and vice-versa, your experiment should behave identically.

Which means there should be no reason for the imbalance that we see. Except, obviously, something had to have happened. Otherwise, everything would just be made of photons, and we wouldn't be having this discussion.

The challenge physicists have now is figuring out what caused that imbalance. And there are a few things they could look for. For example, this mystery could be caused by some reaction that produces more matter than antimatter.

Or it could be caused by a decay reaction that makes antimatter break down more rapidly. Or, more realistically, it could be due to a combination of extremely rare production. And decay processes that add up to a one-in-a-billion difference.

Unfortunately, we haven't come up with a hypothesis that explains anything for sure. But we have uncovered some interesting leads that suggest something weird is going on. Most notably, some experiments have found that one of those three fundamental forces actually treats matter and antimatter a little differently, after all.

It's the weak nuclear force, which governs how atoms decay. And figuring this out was such a big deal that the first researchers to do it earned the 1980 Nobel Prize in Physics. These scientists worked with a particle called a neutral kaon, which is composed of smaller particles called quarks.

Neutral kaons come in two forms, but both are made of one matter and one antimatter quark. One kind is made of a down quark bound to an anti-strange quark. And the other is made of a strange quark bound to an anti-down quark.

For the record, the quarks in the kaon don't annihilate each other because they're not exact counterparts. If these things were made of, say, down and anti-down quarks, that would be a different story. Regardless, in their Prize-winning experiment, the scientists found that something seemed wrong about how these kaons decayed.

Specifically, if you swapped all the matter and antimatter in that experiment, you'd get slightly different results. And since the weak force is responsible for decay, that meant it treated the two things differently. This isn't the last time we observed something like this, either.

In fact, since that discovery, similar results have been found in lots of other particles with different types of quarks. For instance, in 2019, evidence from a large particle accelerator suggested that the weak force treats a particle made of charm quarks differently. We've also discovered similar results with bottom quarks before.

Still, these findings don't exactly answer our question. They're interesting, sure, and the weak force is a major player in physics. Like, I did just say that one thing that could solve this antimatter mystery is if that stuff decayed faster.

But these types of reactions still only involve certain types of quarks. So as great as that would be, they aren't enough to explain how even one extra matter particle in a billion could have survived over the whole universe. Still, someday, maybe they could lead us toward a real answer.

Of course, scientists don't think we'll be able to solve this antimatter problem just by testing and looking at particles in the world around us. Unfortunately… that would just be too easy. Instead, it's likely that we'll need to do a lot more theoretical work to answer this question.

And that will probably mean coming up with a whole new framework for physics. One of the things we might have to overturn is called the Standard Model. It's a really well-tested model that catalogs all the types of particles we know of and also predicts how they should interact.

But even though there's a lot of evidence supporting it and it's been really good at explaining things, researchers have a few reasons to believe there's something beyond the Model, too. Because, among other things, it can't explain how gravity works. And gravity is definitely real, and kind of a big deal.

A bunch of researchers are currently trying to find something called a Grand Unified Theory that will ultimately replace the Standard Model. This theory, if we ever discover it, will be able to describe the three main forces of particle physics as if they were the same force. Scientists think that unified force would have been extremely important in very early universe, so maybe it could tell us what happened with matter and antimatter.

So, maybe that unified force treats the two things differently like the weak force does, but on a much larger scale. That said, we might not need to fully understand a Grand Unified Theory to solve this problem, either. We could figure it out just by making some tweaks or extensions to the Standard Model — although what those might look like… is pretty up in the air at this point.

Honestly, you could follow any of these questions way into hypothetical science territory, but the truth is, we're not really close to getting an answer. We have a ton of fascinating leads and a bunch of ideas we could keep testing, but at the end of the day, this problem with antimatter isn't just about antimatter. It's about how all particles behave and what the early universe was like, and it really highlights the fact that there's a ton we don't know about how things work.

So if we want to understand antimatter, we'll need to roll up our sleeves and really dig into the foundations of physics. There won't be one magical experiment that solves everything, and the first Grand Unified Theory to be widely accepted may not explain it all, either. To figure out what's going on, we'll need out-of-the-box ideas and innovative new experimental designs.

We'll need to know about fundamental forces, and the early universe, and we'll need to be able to round up enough antimatter to do big experiments with it. Also, hopefully careful experiments with it. Even one of these questions is something you could make a whole career out of, and plenty of people have.

But one thing is for sure: Someday, if we do solve this problem, it will be huge. We'll finally be able to answer why the universe looks like it does, and why we're all here. And along the way, we'll have learned so much.

So in this matter-antimatter problem, it's really about the journey just as much as the destination. And this research about things like the weak force and the Standard Model is a big part of that. Thanks for watching this episode of SciShow!

If you want to learn about one way scientists are actively trying to research Grand Unified. Theories, you can check our episode about that over on SciShow Space. [ outro ].