What is cosmic microwave background radiation composed of?
The background radiation is composed by photons.
The early universe was not composed by matter but by electromagnetic radiation at very high energy.
After the expansion the energy of the radiation was distributed on a bigger volume and the density decreases starting to produce the particles (20.000 years after the Big Bang).
Not all the radiation transformed in particles, some of the original electromagnetic radiation is still there. So the microwave background radiation is an electromagnetic wave, in the frequency of the microwave (around 160 GHz) and is like any other radio signal, like the wifi, the mobile phones etc.
Every electromagnetic radiation is composed by photons, like the light that is an electromagnetic radiation.
So the background radiation is composed by photons.