And they never go away except when absorbed by a black hole.
In String Theory, matter takes 9 dimensions to form. Six of these dimensions fold up (also called compactify) into cylinders that cannot be detected. Your and our Abraham in the "Book of Creation" says how that happens, folding left and right in each of three directions to get 6 compactified dimensions.
The compactified dimensions are made of strings of axions. Now when matter is created, it is created in pairs: one electron with one positron. The string of axions connect each electron with its mate the positron, until one of them interacts with the environment. Then the string breaks up and the axions precipitate into space.
This is how the cosmic axions were created. They all precipitated into space when very early in the age of the universe, all the positrons recombined with almost all the electrons. We are what's left over.
So there are more axions in the universe than even photons because each recombination released two photons along with a great number of axions.
The structure of the axions is supposedly that of parallel electric and magnetic fields, according to Cramer as referenced in my paper. But that is his theory. I do not know how he arrived at that conclusion.
I could understand that the compactification of 16 of the original 26 dimensions of string theory could be made up of force fields like E and H, as forces (photons) can exist in 26-D as the unified field.
But 10-D matter contains charge and I would expect that axions formed from the recomdination of charged particles would carry some kind of charge rather than just fields.
Hope that is helpful?