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Where is all the antimatter? Finding out what caused the matter-antimatter asymmetry

This article is an introduction to the unsolved problem of matter-antimatter asymmetry in particle physics and cosmology. Why is the amount of matter in the universe so much greater than the amount of antimatter? This article answers the question "what is antimatter?" and reviews the scientific theories that have been suggested to explain the asymmetry.

What is antimatter?

To answer this, we first need to take a step back and ask: what is ordinary matter? In simple terms, it is the stuff that we see around us everyday. Speaking more scientifically, it consists of vanishingly small particles that join together to form atoms, which in turn stick together in various configurations to make up every object on Earth. The very smallest building blocks – such as quarks and electrons – are known as elementary particles.

For each type of elementary particle, there also exists a type of antiparticle which has the same mass and spin properties as its particle sibling, but opposite charge. The existence of antiparticles was proposed by Paul Dirac in 1928, and the first one was observed four years later by Carl D. Anderson, who was looking at tracks left by particles in a cloud chamber. Anderson saw a track that looked exactly like the path of an electron except for one detail: it curved anticlockwise instead of clockwise in the magnetic field of the cloud chamber, which indicated that the particle responsible for the track was positively rather than negatively charged. The object responsible for the track had to be a new kind of particle - a positive electron, or "positron". Anderson was awarded the Nobel Prize for Physics in 1936 for this discovery.

The first image of an antiparticle, by Carl D. Anderson (1905-1991) (Physical Review, Vol.43, p. 491)

The first image of an antiparticle, by Carl D. Anderson (1905-1991) (Physical Review, Vol.43, p. 491)


Antiparticles such as positrons are extremely rare in our universe. They are found in cosmic rays – streams of particles from space which continually stream through our atmosphere – but in numbers which are vanishingly small compared to the billions of electrons that are present in a single grain of sand. And this is where the mystery arises. Even the seemingly chaotic interactions between elementary particles follow rules: in all processes that have been observed to date the “baryon number”, defined as the number of particles minus the number of antiparticles in the interaction, is conserved. This means that if the universe started with equal numbers of particles and antiparticles, their numbers should always stay equal.

So how is it possible that we have ended up with a universe in which the amount of antimatter is almost insignificant compared to the amount of normal matter?

Explanation 1: That's just the way it is

We could suppose that the Universe simply came into existence with a large preponderance of matter over antimatter, and that this initial imbalance has simply perpetuated. This is a deeply unsatisfying explanation. Why should our universe – which otherwise displays a remarkable degree of symmetry – have been born with such a blatant asymmetry built in?

Explanation 2: Conservation laws aren't always obeyed

An alternative explanation is to suppose that the extremes of temperature and pressure that existed in the first few seconds after the Big Bang allowed processes that violate the conservation of baryon number to take place. We do not see these processes taking place today because the energy density of the Universe is simply too low to allow them. Theoretical physicists across the globe are currently working on extensions to the standard model of particle physics that describe such a mechanism for the creation of particles.

Explanation 3: The antimatter isn't missing – just hidden

Another suggestion is that the missing antimatter is in fact not missing after all, but rather that it is hidden, perhaps in regions of the Universe that are too far away for us to see. Some scientists point to the heavy clouds of dark matter which lurk within galaxies, undetectable except by the gravitational effects of its mass on other astronomical objects, and suggest that this dark matter may in fact be made of antiparticles.

  • The Evidence for Dark Matter
    Dark matter is the name given to the matter that holds together the Milky Way and other galaxies. It cannot be seen using telescopes as it does not emit light or other radiation. However, there is significant evidence for its existence...

topquark works as a researcher in theoretical particle physics and blogs about research at The Particle Pen.


Shinto on February 03, 2012:

recently , it was heard that , the result from LHC experiment shows that the particles were 10 times the number of antiparticles during Big Bang .

is it true ?

can you tell us more about it ?

Xavier Nathan from Isle of Man on July 07, 2011:

You have done another great job on this and I think your articles are the perfect length for the subject matter you treat. I love reading your hubs and in time you will see a lot more people appreciating what you have to say and the way you say it.

Ron Hooft from Ottawa on June 17, 2011:

Yes. We have a plethora of interpretations and not enough evidence to warrant saying any of them reflect reality.

Thanks for that explanation. It does make sense. After all the universe is a big place.

Could dark matter not just be the fabric of space? After all we know it is not empty, but full of quantum activity. Could this not be the cosmological constant rather than something new? I know we see lensing etc. But couldn't that be explained by other factors than a new dark matter?

The other thing I wanted to pick your brain on is, do we know for sure that all particles are born twins, matter/anti matter? In other words is symmetry a fact?

topquark (author) from UK on June 15, 2011:

I try not to go on too much... that's sometimes difficult because there's always more to say.

Not necessarily. An antiparticle will annhilate with its partner: eg. electrons with antielectrons (positrons), neutrinos with antineutrinos, etc. If the antiparticles making up dark matter are the partners of particles that aren't common in those regions of the universe, there won't be any annihilation. It could be that after the universe formed some regions of space contained more dark matter particles than dark antiparticles, so that when they annihilated there was still an excess of dark matter left, and some regions where the opposite was true so that it was the dark antimatter that was left.

Also there is gamma radiation arriving from some areas of the sky that no-one has determined the source of, which could possibly be the result of annihilation, although there are other equally plausible explanations.

The short answer is: no-one knows. This is all very speculative. There are loads of theories about how particle physics works beyond the standard model and not yet enough evidence to differentiate between them. I find it fascinating to read about the possibilities though.

Ron Hooft from Ottawa on June 14, 2011:

Wonderful hub. My only complaint being your hubs are too short. ;)

If dark matter were antimatter would we not see matter/antimatter annihilation in the regions of space we think the dark matter is in?

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