After the Big Bang, the universe was initially filled with a high-energy plasma consisting of photons, electrons, and atomic nuclei. This state lasted for about 380,000 years until the universe cooled down enough for atoms to form, a period known as recombination.
During the initial stages of the universe, immediately after the Big Bang, the dominant form of radiation was electromagnetic radiation, primarily in the form of high-energy photons. These photons were scattered frequently by the charged particles in the plasma, making the early universe opaque to light. This period is often referred to as the "era of radiation domination."
However, as the universe expanded and cooled down further, the energy densities of electromagnetic radiation and nuclear radiation (primarily in the form of free neutrons and protons) changed. Approximately a few minutes after the Big Bang, a process called Big Bang nucleosynthesis occurred, during which light atomic nuclei, such as hydrogen, helium, and lithium, were formed. This process requires a high density of free neutrons and protons.
So, to answer your question, electromagnetic radiation (photons) was the dominant form of radiation immediately after the Big Bang, followed by the formation of atomic nuclei and the emergence of nuclear radiation. The formation of light atomic nuclei took place after the era of radiation domination.