Quoting [this page from CERN](https://home.cern/science/physics/matter-antimatter-asymmetry-problem):
>If matter and antimatter are created and destroyed together, it seems the universe should contain nothing but leftover energy.
>
>Nevertheless, a tiny portion of matter – about one particle per billion – managed to survive. This is what we see today.
But if the difference is a billion universe’s worth of matter-antimatter annihilations, shouldn’t there be a billion times more of this ‘leftover energy’ in the universe than we see today?
In: Physics
When matter and antimatter meet up and annihilate, they create a very high energy gamma ray.
One of the weird features of gamma rays is that they have enough energy to spontaneous transform into an electron and a positron pair (pair production) and if your gamma ray has enough energy it can transform into heavier pairs like a proton and antiproton.
So you start with a universe with a lot of raw energy which transforms into matter-antimatter pairs, they annihilate leaving you with a tiny bit of matter and most of the energy back as energy. That energy transforms again into matter-antimatter pairs which annihilate and leave a small amount of matter survivors. Repeat for a few thousand cycles and you end up with a fair amount of matter and a limited amount of free energy in the system.
Mass and energy can be transformed between each other very freely in high energy states like the early universe
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