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The Standard Model of Particles Physics is   the best theory we have for how the  universe works on subatomic scales. It’s not even a close competition.

Like Lebron versus a  snail in a dunking contest. No other theory is as good at describing the   particles that make up reality and  how they interact with one another. But despite how impressive the  Standard Model is, it is also wrong.

And it’s wrong in ways that leave some absolute  whopper mysteries for scientists to solve. So here are five of the biggest  problems with the Standard Model,   as well as some ways physicists  are trying to patch the cracks. [♪ INTRO] In 2013, two physicists won  the Nobel Prize in Physics for   predicting the existence of a single  subatomic particle: the Higgs Boson. They made their predictions back in the 1960s,   but the Higgs Boson wasn’t officially discovered  through experimental observations until 2012.

The Higgs Boson helps give all the  other subatomic particles their mass,   so the Standard Model just  doesn’t make sense without it. Hence why physicists spent  so long trying to find it. In the annals of particle physics  history, this was just one of   many times the Standard Model described a  thing before we knew it actually existed.

And by “we” I mean humanity as a collective. Because, you know, some of us were too busy not  being born, or being teenagers, to contribute. But for the Higgs Boson, there was one major  problem: The particle that CERN discovered   had a mass of 125 gigaelectron volts, or  roughly 130 times the mass of a proton.

That might sound kinda big for  a single subatomic particle. But it’s too light. Much too light.

It might be as many as 34  orders of magnitude too light. That’s ten million billion billion  billion times what it “should” be. Now, I had to pull out the scare quotes because  the Standard Model can’t really predict the mass   of the Higgs Boson at all, because the equation  for its mass depends on a number that’s unknown.

It’s called the UV cutoff, and it’s  the point at which the energy levels   of the thing we’re observing are  too high for our math to work. But like I said, we don’t actually  know the exact value of the UV cutoff. It’s just vaguely…over there, somewhere.

Since we don’t actually know  that number, our theories have   to work for all possible values of the  cutoff, including really enormous ones. Which means the equation would need  equally enormous negative values   somewhere else to balance that out and give us  the mass we’ve observed for the Higgs Boson. What could cause this uber  convenient cancellation?

We have no clue. And unfortunately, that means we’ve  run into a case of finetuning. And finetuning gives physicists the willies.

It’s not impossible that nature could   so carefully and so precisely balance  the energy checkbook of the Universe. But when a theory requires such  a precarious balancing act,   physicists get jumpy and try searching  for something entirely different. One of the leading ideas to explain   the mysteriously-normal-sized  Higgs boson is supersymmetry.

We don’t have time to get into the physics of  exactly how it helps, but it poses there’s a   whole set of particles lurking at energies higher  than we’ve been able to create in experiments. These new particles would be paired  with Standard Model counterparts. Like traditional quarks would  be partnered with supersymmetric   squarks, and leptons would be  partnered with, yes, sleptons.

These new guys have some properties that are  opposite from their Standard Model partners,   and this opposite-ness shows up  in our equations as a minus sign. So if supersymmetry particles really do exist,   the calculation for the Higgs mass would better  agree with what we actually observe…without   scientists having to just add a number  without knowing why it should be there. There’s still no solid evidence of supersymmetry,   so physicists are tracking down  other potential solutions, too.

At CERN, they’re even searching  for more Higgs particles,   because some new theories allow for  more than one type of Higgs Boson. But for now, the finetuning problem persists. And it persists for our next  problem with the Standard model.

Because as it turns out The Universe is expanding   way too slowly compared to what  the Standard Model predicts. And by “way”, I mean as much as a 1  followed by 120 zeros times too slowly. This is sometimes called “the worst  prediction in the history of physics”,   because this is just an unbelievable degree  of wrongness compared to our observations.

But let’s back up in case you missed the  newsflash from the late 90s: The Universe is   not just expanding…which we’ve known for almost  a century, now…that expansion is accelerating. This discovery also earned  a Nobel prize, in 2011. And this is the last time I’ll  mention the Nobels, promise.

In other words, the Universe is  perpetually getting bigger, faster. So something must be fueling that expansion. Scientists call this mysterious  “something” dark energy.

And they have no idea where to go from there. One of the simpler hypotheses relies  on the Standard Model’s assumption   that the Universe is filled with vacuum energy:   basically, the energy that comes with  everything in the universe just… existing. The problem is the total amount of vacuum  energy in the observable universe should   be causing the Universe to expand so  fast, that nothing of substance could exist.

Even individual atoms would struggle  to form in such an environment,   let alone things made of many atoms. Like stars, and planets, And those tiny little porcelain trinkets that your grandparents maybe collected. And, to be honest, I think they were kind of on to something.

Like, there's something so fun about having just, like, a little tiny, delicate trinket in your hand and putting them on the shelf and stacking them up. Then you can just sit back and look at all of them and... At best, theoretical physicists  can reduce that 10^120 discrepancy   between what the Standard Model predicts and  astronomers actually observe down to 10^60.

It involves some of that supersymmetry I talked   about earlier, but it's nowhere near helpful enough. For now, we’ve run into  another finetuning problem. It seems like there should be some other  number in the appropriate equations with   a power of negative 120 that cancels out  almost all of the predicted vacuum energy.

And again, scientists would rather  find a more “natural” solution. But when it comes to potential solutions,  they’re still throwing spaghetti at the wall. And so far, not much has stuck.

One set of ideas involves quantizing space. Most of our theories are built on the  assumption that the universe is a smooth,   continuous fabric, even down to  the smallest scales of reality. But maybe that’s wrong.

Instead, spacetime might be made  of discrete, quantized chunks. Like pixels. This would alter the calculations we’d  have to do for the strength of vacuum   energy, and we might wind up with an  estimate much closer to observations.

Another possible solution relies on re-imagining   the structure of the universe as  a sort of frothing quantum foam. On the very smallest scales, spacetime  could actually be full of holes that pop   in and out of existence, like  the foam on a pint of beer. These fluctuations could disrupt the  Standard Model’s vacuum energy calculations,   and cause the vacuum energy to be  far less than current predictions.

While both of these solutions are technically  allowed within the boundaries of modern physics,   they both have one major disadvantage:  they’re really hard to test, because   they involve measuring distances that are 20  orders of magnitude smaller than a proton. Some scientists doubt we’d  ever be able to test them. But that’s no reason to quit while  we’re behind, because this isn’t the   only time the Standard Model has thrown us  something that’s nearly impossible to study.

When it comes to neutrinos, the  Standard Model has very little to say. It tells us there are three kinds, or flavors, of  neutrino, and their three antimatter counterparts. It tells us they’re all electrically  neutral, hence their name.

And it tells us they’re impossibly tiny,  even where subatomic particles are concerned. But it can’t tell us how tiny, either in terms of  the space they occupy, or in terms of their mass. In fact, the Standard Model originally  predicted neutrinos don’t have mass at all,   except we know from experiments  that they very much do.

If they didn’t have mass, neutrinos wouldn’t be  allowed to occasionally swap what flavor they are. Which physicists figured out over two decades  ago, and got a Nobel Prize for it in 2015. Ok, sorry.

That was the last time I'm going to reference the Nobel Prize… There are so many mysteries surrounding  neutrinos, and SciShow keeps covering them. We’re still not done, because in this episode,   let’s focus on a decades-old question  mark called the Gallium Anomaly. Buried beneath the mountains on the  border between Georgia and Russia   is the Soviet-American Gallium  Experiment, or SAGE, for short.

The actual experiments involved bombarding a   sample of the metal Gallium  with a bunch of neutrinos. Every once in a while, a neutrino would collide   with a neutron inside a Gallium atom,  and turn the neutron into a proton. Since it’s the number of protons  that determines what element you are,   this collision also turned that  Gallium atom into Germanium.

However, when the researchers  actually counted up their Germanium,   they found about 20% less than they predicted. Those predictions, of course, were based off   the Standard Model’s version of  subatomic particle interactions. Despite their best efforts, SAGE  researchers have been scratching   their heads over this discrepancy since the 1990s.

In 2014, they even built a follow up experiment,   called the Baksan Experiment  on Sterile Neutrinos, or BEST. One of its goals was to check whether the Gallium  Anomaly could be chalked up to experimental error,   like miscounted atoms or miscalculations in the  expected rate of neutrino-neutron collisions. The good news?

The experimental results held up. They even improved on SAGE’s in terms of quality! But that also means bad news:  The researchers were left with   the exact same questions about  why this anomaly exists at all.

So far, the only maybe-sorta-feasible solution  that scientists have proposed is a new, fourth,   flavor of neutrino that the Standard Model  doesn’t account for: sterile neutrinos All neutrinos are notoriously evasive; a  neutrino could pass through a lightyear   of lead with only a 50% chance of  bumping into anything along the way. But sterile neutrinos would be even more ghostly. The only way they could interact  with anything was through gravity,   because like the three traditional  flavors, it would need to have mass.

That way, they could also participate  in all that flavor swapping. Maybe that Germanium is missing because 20% of   the neutrinos happened to turn into  sterile neutrinos at the wrong time,   and refused to interact with the Gallium  at all…instead of just incredibly rarely. Unfortunately, before you get too excited about   the possibility of sterile  neutrinos, there is a catch.

The kind of sterile neutrino you’d  need to solve the Gallium Anomaly   is a much lighter version than most  scientists are currently looking for. Other experiments have searched for light   sterile neutrinos in this mass  range, and have come up empty. So if sterile neutrinos do exist…which  physicists have proposed to answer   other problems with the Standard  Model…they might all be super heavy.

In fact, there’s a new neutrino experiment called  JUNO that came online in China in August 2025. They’re planning to precisely  measure neutrino flavor swapping. And maybe, the info they gather can help us  fix the next problem with the Standard Model.

But before we get to that, we have the  standard problem of paying the bills. So here’s a quick ad. Thanks to Squarespace for  supporting this SciShow video!

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Do you exist? Congratulations, you’re evidence  that the Standard Model is wrong. Because at the dawn of our universe,   both matter and antimatter should have been  popping into existence at the same rates.

But if matter and antimatter twins annihilate  when they get too close to each other,   then all the universe should have been filled  with for the rest of time is a bunch of light. Well, technically there’d still be a  bit of quantum particle shenanigans. But I mean no galaxies, no stars, and no tiny little porcelain trinkets like the ones your grandparents might have collected.

And I'm honestly hoping that I inherit my grandparents' collection Because I think that'd be really special and sentimental in addition to the ones that I thrift... It looks like matter won the  battle, although not by much. For every one billion antimatter particles,   just one billion and one matter  particles must have been created.

But before we can theorize about how this  came to be, we need to back up a little. There’s a symmetry in nature that  is…for the most part…respected. It’s called charge-parity, or CP symmetry.

Physics should work the same if you  flip all of the particles with their   opposite-charged anti-particles and also  flip the definitions of left and right. After all, positive versus negative and left  versus right are just human terms we made up. Those definitions should not  affect the physics going on.

But I do keep saying “should”...because   researchers have discovered a couple  situations that violate CP symmetry. Antimatter fundamentally behaves  differently than matter does. They aren’t exact counterparts.

And despite what you might think, the  Standard Model is kind of okay with that. In fact, it predicts CP violation could  happen in certain weak force interactions. In other words, situations where unstable  subatomic particles are decaying into other stuff.

However, the amount of CP violation predicted by   the Standard Model can’t account for all  the asymmetry that allows us to exist. So far, the only place we’ve observed CP violation  is in the behaviors of particles made from quarks,   a class of subatomic particles that includes  the stuff inside protons and neutrons. While the Standard Model is good at  explaining this particular case of CP   violation, we need something more to fully  explain the matter-antimatter asymmetry.

So guess who’s back to maybe save the day? Neutrinos! Physicists suspect that the  violation they observe in   quarks also happens with a different  class of particles called leptons.

The most famous lepton is the electron,  but neutrinos are leptons, too. We don’t have time to get into the details, here. But if scientists found CP violation in neutrinos,   it could unlock the theory for a process called  leptogenesis, which provides a mechanism for   generating more matter than antimatter  during those early years in the Universe.

Or maybe, we don’t need neutrinos at all! The answer to our asymmetry problem  might be hidden with the quarks. Because according to a paper published  in 2025, experiments conducted at CERN   found far more CP violation on the quark  side than had previously been observed.

Or maybe, there’s something else completely  beyond the Standard Model to uncover. But in terms of elements  missing from the Standard Model,   we have to end on what’s  arguably the most glaring. Gravity isn’t in the Standard Model at  all…perhaps the most obvious of physics’   four fundamental forces in our day-to-day lives.

Not that the electromagnetic,   weak, and strong nuclear forces aren’t  critical to our existence, of course. But babies learning to lift  their heads know about gravity. It’s so different because, at the moment, our  best understanding of gravity comes from the   theory of general relativity, in which  gravity isn’t technically even a force.

Instead, GR tells us it’s a side effect of mass  creating curves in the fabric of spacetime. So gravitational force, as we currently  understand it, is more geometric in nature. GR’s version of gravity also  lacks a messenger particle:   a particle that ferries that  force through space and time.

The electromagnetic force has the photon, the  weak nuclear force has the W and Z bosons,   and the strong nuclear force has gluons. Each is described thoroughly  by the Standard Model. Some researchers have proposed the  existence of gravitons for gravity,   but there’s no evidence that they really exist.

Even if they did, there’s still another pretty   glaring mystery about gravity  relative to the other forces. It is so much weaker. A cheap fridge magnet defies the gravitational  pull of the entire Earth, easy peasy.

And the gravitational pull between two electrons   is about 43 orders of magnitude weaker than  the electromagnetic force pushing them apart. Sure, that's no 10^120, but  it’s still an enormous gap. One way to bridge this metaphorical canyon is by   adding extra dimensions to  our theories for spacetime.

The Universe we experience is  made of three spatial dimensions,   and a fourth dimension for time. But maybe spacetime has additional dimensions   curled up and hidden because we’re  just too darn big to perceive them. Imagine walking a tightrope  across our metaphorical canyon.

You can only move forward or backward. But if an ant decided to  follow along for moral support,   they’d be small enough to  also move left and right. Perhaps to better evade your feet  as you inevitably lose your balance.

If spacetime really does have  extra, hidden dimensions,   they’d only be observable using  very very large amounts of energy. So much energy, perhaps, that no  experiment on Earth could ever reveal them. But if these dimensions exist, then gravity  might be much stronger than we think.

It would just have to spread itself across  the many different spatial dimensions. That would dilute its full strength so  that we, in our 3D subsection of space,   would only observe a miniscule fraction of it. Or remember those theories that quantize space and  might help us understand the universe’s expansion?

Well they could also help explain how  gravity fits in with the Standard Model. But we’re technologically just as far  from understanding quantum gravity as   we are from finding any hidden dimensions. There is one argument that could  explain not just why gravity is so weak,   but many of the problems we’ve  talked about in this episode.

The Anthropic Principle says that the Universe is  the way it is because we’re here to witness it. If conditions were just a little bit different…if  gravity were just a bit stronger, if the universe   were expanding just a bit faster, if all the  matter and antimatter completely annihilated   at the beginning of the universe…then we  couldn’t ask why because none of us would exist. So it has to be this way.

That sounds a lot like fine-tuning to me,   with a bit of circular reasoning  thrown in there for good measure. It also sounds like a boring way to  answer some really interesting questions. There’s plenty to learn just  by searching for answers.

And getting unexpected results is  part of the excitement of science! Maybe someday we’ll have updates on these  five problems with the Standard Model. And when the news does break, you can be sure  there’ll be a SciShow video about it. [♪ OUTRO]