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Message: <blockquote data-quote="Umbran" data-source="post: 6687546" data-attributes="member: 177">All around you!  We are talking about events at a time when the universe was extremely dense, just a plasma of particles.  Lots of particles and antiparticles tooth-by-jowl, so to speak.  When the particles annihilate, what you typically get* is photons**.  With things so dense, those photons get immediately absorbed by particles of matter***, and re-emitted.Eventually, as the universe expanded, atoms formed, and the universe dropped to a density where photons could fly free without running into stuff.  What photons were left that didn't run into stuff to be absorbed became what we now call cosmic microwave background radiation.*You sometimes get particle-antiparticle pairs instead, but then those annihilate, too, because everywhere you go there's stuff to annihilate with, so eventually you get back to photons.** Freyar may correct me if I am wrong - this may be happening before the electromagnetic force and weak nuclear force decoupled, so we would then be looking at W and B bosons, carrying the electroweak force of the time, but the rest is about the same.***We note there that "matter" in this case may also include so-called "dark matter" - while today it doesn't interact much, back in the day or high density it may have been a major contender- so some of that energy may be tied up in stuff you can't see now.</blockquote>