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This question already has an answer here:

I've tried to find an answer for this but couldn't find one. Whats the problem with matter being created with little or no antimatter

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marked as duplicate by Ben Crowell, Kyle Kanos, Brandon Enright, ACuriousMind, Ali Oct 3 '14 at 21:21

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ I voted to leave open, because the proposed duplicate answers the question only for experts who understand the whole picture and terms such as "baryogenesis" or "Planckian time" - and the answer is not direct. A more lay-oriented and direct answer seems desirable. $\endgroup$ – Void Oct 3 '14 at 20:10
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The interconversion of matter and energy is described by quantum field theory. If you're interested the question What keeps mass from turning into energy? is on this subject.

The particular quantum field theory that describes our universe is called the Standard Model, and there are three important symmetries that apply to the standard model - charge symmetry, parity symmetry and time symmetry.

It turns out that neither charge symmetry nor parity symmetry are obeyed on their own, but the combination of charge and parity symmetry, CP symmetry, is almost (but not quite) obeyed. Matter and anti-matter are related by CP symmetry, so any CP symmetric process must create equal amounts of matter and anti-matter - creating different amounts of matter and anti-matter would violate CP symmetry.

So if CP symmetry were an exact symmetry there wouldn't be any matter in the universe. The processes that create matter would have to create equal amounts of matter and anti-matter, and these would annihilate to leave a universe containing just photons.

However it turns out that the in the Standard Model CP symmetry is almost but not quite an exact symmetry, so there can be processes that create different amounts of matter and anti-matter and therefore result in a net creation of matter. However these processes create too little matter to explain all the matter we see around us. There must have been some other process that we don't currently understand which created the matter we see today.

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  • $\begingroup$ This doesn't answer the OP's question, which is why the universe couldn't just always have had an asymmetry, ever since the big bang. $\endgroup$ – Ben Crowell Oct 3 '14 at 17:54
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Creating only matter without antimatter would violate several conservation laws. Mainly the electric charge conservation and color charge conservation. If you create a lot of electrons without antielectrons (positrons), you create a lot of negative electric charge out of nothing. That is not allowed by the conservation laws. Similarly, you cannot create a lot of color charge without color anti-charge, when creating quarks.

The whole subject is more complicated and you should look for example at the CPT symmetry.

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  • $\begingroup$ This doesn't answer the OP's question, which is why the universe couldn't just always have had an asymmetry, ever since the big bang. $\endgroup$ – Ben Crowell Oct 3 '14 at 17:55

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