Baryon asymmetry

The baryon asymmetry problem in physics refers to the apparent fact that there is an imbalance in baryonic matter and antibaryonic matter in the universe. Neither the standard model of particle physics, nor the theory of general relativity provide an obvious explanation for why this should be so; and it is a natural assumption that the universe be neutral with all conserved charges.[1] The Big Bang should have produced equal amounts of matter and antimatter, as such, there should have been total cancellation of both. In other words, protons should have cancelled with antiprotons, electrons with antielectrons (positrons), neutrons with antineutrons, and so on for all elementary particles. This would have resulted in a sea of photons in the universe with no matter. Since this is evidently not the case, after the Big Bang, some physical laws must have acted differently for matter and antimatter.

There are competing hypotheses to explain the matter-antimatter imbalance that resulted in baryogenesis, but there is as yet no one consensus theory to explain the phenomenon.

Possible explanations
CP violations

Most explanations involve modifying the standard model of particle physics, to allow for some reactions (specifically involving the weak nuclear force) to proceed more easily than their opposite. This is called "violating CP symmetry" in weak interactions. Such a violation could allow matter to be produced more commonly than antimatter in conditions immediately after the Big Bang. However, as of yet, no theoretical consensus has been reached regarding this, and there is no experimental evidence of an imbalance in the creation rates of matter and antimatter.[2]
Regions of the universe where antimatter predominates

Another possible explanation of the apparent baryon asymmetry is that there are regions of the universe in which matter is dominant, and other regions of the universe in which antimatter is dominant, and these are widely separated. The problem therefore becomes a matter/antimatter separation problem, rather than a creation imbalance problem. Antimatter atoms would appear from a distance indistinguishable from matter atoms, as both matter and antimatter atoms would produce light (photons) in the same way. Only in the border between a matter dominated region and an antimatter dominated region would the antimatter's presence be detectable, as only there would matter/antimatter annihilation (and the subsequent production of gamma radiation) occur. How easy such a boundary would be to detect would depend on its distance and what the density of matter and antimatter is along it. Presumably such a boundary would lie (almost by necessity) in deep intergalactic space, and the density of matter in intergalactic space is reasonably well established at about one atom per cubic metre.[3][4] Assuming this is the typical density of both matter and antimatter near a boundary, the gamma ray luminosity of the boundary interaction zone is easily calculated. Approximately 30 years of scientific research have placed boundaries on how far away, at a minimum, any such boundary interaction zone would have to be, as no such zones have been detected. Hence, it is now considered unlikely that any region within the observable universe is antimatter dominated.

At least one more major scientific study, called the Alpha Magnetic Spectrometer, is planned that would, among other things, advance our capability of detecting very distant antimatter dominated regions.[5]

Another possibility is that antimatter dominated regions exist within the universe, but outside our observable universe. Inflationary cosmology models suggest that there may be more to the universe than can be seen from the Earth, if only for the simple reason that the universe isn't old enough for light from the most distant parts of the universe to have reached us yet. If so, radiation from the boundary of matter and antimatter dominated regions may simply still "be on its way" to Earth, and so cannot be observed.

Yet another possibility is that antimatter repels ordinary matter rather than attracting it gravitationally. This would prevent observable interactions (see Motivations for antigravity); however, it appears more likely that matter and antimatter attract each other gravitationally (see Antimatter gravity debate).

Electron dipole moment

There is a theory which suggests the existence of a dipole moment in electrons which would result in an unequal distribution of the electron's negative charge. Such a property would allow matter and antimatter to decay at different rates. A new ceramic material, called europium barium titanate, was created to test this theory, with experiments being underway as of 2010.[6]

Jerome Friedman on the matter antimatter asymmetry.