https://phys.org/news/2018-07-antimatter-plasma-reveals-secrets-deep.html

Researchers believe that these enigmatic, highly-energetic pulses of radiation are produced by bursts of electrons and their antimatter twins, positrons. The universe was briefly filled with these superheated, electrically charged particles in the seconds that followed the Big Bang before all antimatter vanished, taking the positrons with it. But astrophysicists think the conditions needed to forge positrons may still exist in the powerful electric and magnetic fields generated around pulsars.

"These fields are so strong, and they twist and reconnect so violently, that they essentially apply Einstein's equation of E = mc2 and create matter and antimatter out of energy," said Professor Luis Silva at the Instituto Superior Técnico in Lisbon, Portugal. Together, the electrons and positrons are thought to form a super-heated form of matter known as a plasma around a pulsar.