What Happened to the Energy from Matter/Anti-Matter Annihilation?

[phetuspolice]

I've read how in the early early universe, there were massive amounts of collisions between matter and anti-matter, about a billion of each annihilated to every atom left over.
My question is, what happened to all that energy?

 

[mathman]

I've read how in the early early universe, there were massive amounts of collisions between matter and anti-matter, about a billion of each annihilated to every atom left over.
My question is, what happened to all that energy?

Without going into a lot of detail, it is the cosmic microwave background.