9 trillion kilograms of antimatter a second?

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Antimatter is rare in this Universe, but the Universe is a pretty big place, so even small quantities can add up fast. In our galaxy alone, there's a steady bath of radiation that indicates positrons are constantly running into their electron anti-partners and annihilating them. Over something the size of a galaxy, that means there are lots of the positrons around. Estimates have it that 9.1 trillion kilograms of antimatter are being destroyed each second.

Where's it all coming from?

 

https://arstechnica.com/science/2017/05/explaining-why-our-galaxy-produces-so-much-antimatter/

 

Could this have something to do with "Dark Matter"?