The universe contains roughly 1080 particles and 1089 photons. Why are there so many more photons than particles? To answer this fundamental question, we must explore the idea of symmetry in nature. Symmetry means that phenomena that appear distinct share a similar basis. (In geometry, a sphere is the most symmetric shape because it looks the same from every direction. By contrast, a lumpy rock looks different from every direction.) Scientists look for symmetry in order to unify the diversity of nature. Symmetry also has an aesthetic aspect — it expresses balance and harmony.
There are several important examples of symmetry in physics. Electricity and magnetism appear to be quite different phenomena. Yet Michael Faraday showed over 200 years ago that a moving magnet could generate an electric current and that a changing electric current could generate a magnetic field. The interplay between electric and magnetic forces finds its most beautiful expression in electromagnetic waves. Light is our everyday example of symmetry between electricity and magnetism. Particles and waves appear to be quite different phenomena. Yet although light has obvious wave properties, it also behaves like a tiny particle of energy called a photon. Equally, subatomic particles are "fuzzy" in the quantum view of nature and often have the properties of waves. The last example is the famous result of Einstein: E = mc2. This equation expresses the fact that mass and energy are not separate and distinct. Mass is a "frozen" form of energy.
Although symmetry may be present in our theories or the equations that express the theories, it is not always obvious in the universe we live in. The equation E = mc2 implies that mass and energy are interchangeable. Yet it is only in the extreme conditions inside stars that we see even a small percentage of mass turned into energy. In the everyday world, we never see the reverse process; light does not spontaneously turn into particles. In our universe, there is an enormous asymmetry between radiation and matter. Photons outnumber particles by about a billion to one. Of course, these photons are so feeble in energy now that we can barely detect their effects.
Symmetry is concealed from us in the nature of matter. Antimatter is material whose particles have an opposite set of quantum properties. Think of antimatter as the mirror image (or ghost) of matter. In the microscopic world of the quantum, every particle has an equivalent antiparticle. Charged particles have antiparticles with the opposite charge. For example, the antiparticle of the electron is the positively charged positron, and the antiparticle of the proton is the negatively charged antiproton. The counterpart of the neutron is the neutral antineutron, and the counterpart of the neutrino is the antineutrino. The photon is its own antiparticle. The laws of physics place matter and antimatter on an equal footing, so why is the ordinary world composed overwhelmingly of matter? When an antiparticle is created in the laboratory, it cannot survive long before encountering a particle. The result is that both particles disappear in a flash of gamma rays.
Astronomers have wondered whether there are large concentrations of antimatter somewhere in the universe. There are no antimatter stars anywhere near the Sun — they would react with the interstellar medium to produce far more gamma rays than are observed. Astronomers have even ruled out distant antimatter galaxies made of antimatter stars. Galaxies and anti-galaxies would currently be separated by the depths of space, but in the past they must have been closer together and we do not see gamma rays from their interactions redshifted by the expansion to lower energies. On all scales we can observe, antimatter is rare or absent. The universe is made of matter.
The symmetries of nature were more readily apparent in the early universe. At a high enough temperature, mass and energy were freely interchangeable. Equality was established between the creation and annihilation of particles and between matter and antimatter. The present day universe is almost entirely made of radiation and matter. Antimatter can only be produced in tiny, ephemeral amounts. In round numbers the universe contains roughly a billion photons for every particle, and no antiparticles. However, particles and antiparticles were created and destroyed in the early universe in equal numbers. One millionth of a second (10-6 s) after the big bang, the temperature was an enormous 1014 K, and gamma ray photons had enough energy to create particle/antiparticle pairs of many kinds. By about a second after the big bang, cosmological expansion red shifted the radiation to a low enough energy that particles and antiparticles could not be created.
If there had been exactly equal numbers of particles and antiparticles just after the big bang, they would have all disappeared in pairs, leaving a universe filled with only radiation! Thus, there must have been a tiny excess of matter over antimatter in the very early universe. For every billion antiparticles, there must have been a billion plus one particles. After particles and antiparticles paired up to create a flood of gamma rays, a slight residue of matter remained. Now, billions of years later, the original gamma rays of the big bang have been red shifted to feeble microwaves. Although these photons have low energy, each cubic meter of space contains about 109 of them. Most of the particles in the universe long ago annihilated with their corresponding antiparticles, and the residue of neutral matter has formed all the gravitational structure in the universe. In other words, our existence depends on a tiny asymmetry between matter and antimatter in the very early universe!
Where did the asymmetry between matter and antimatter come from? One possibility is that the universe started that way; it as built into the initial conditions. Most people consider this a rather sterile hypothesis. The most viable explanations involve unification physics, or mechanisms that result from the decoupling of fundamental forces very early in the history of the universe. This is a very active field of theoretical physics, with no immediate resolution in sight.