Galaxies & Cosmology
The early Universe was filled with a quark-gluon plasma, in which all kinds of particles and anti-particles — quarks & anti-quarks, leptons & anti-leptons — were flying around along with force-carrying particles (bosons).
Within this hot, dense universe, particle-&-antiparticle pairs were colliding and annihilating to become energy (photons), and also pairs of photons were interacting to convert into particle-antiparticle pairs.
A particle and its anti-particle are colliding and producing a burst of gamma-ray energy (photons in red squiggly lines, in the animated image below). This is called matter-antimatter annihilation.
Universe Adventure — LBL Physics