Light, as we know it, travels through the universe in the form of electromagnetic radiation. Electromagnetic radiation encompasses a broad range of wavelengths, and visible light is just a small portion of this spectrum. The full spectrum includes various types of radiation, such as radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.
When we talk about light from 13.7 billion years ago, we are referring to the cosmic microwave background radiation (CMB). The CMB is the remnant radiation from the early universe, specifically from about 380,000 years after the Big Bang when the universe had cooled enough for atoms to form.
During that time, the universe was in a hot, dense state filled with a plasma of particles, including protons, electrons, and photons. However, as the universe expanded and cooled, the plasma eventually became transparent, and the photons could travel freely without being continuously scattered by charged particles.
The photons of the CMB, also known as relic radiation, have undergone significant redshift due to the expansion of the universe over the past 13.7 billion years. This means that the wavelengths of the CMB photons have been stretched as the universe expanded, shifting them toward longer wavelengths. As a result, the CMB primarily exists in the microwave portion of the electromagnetic spectrum, with an average temperature of about 2.7 Kelvin (-270.45 degrees Celsius or -454.81 degrees Fahrenheit).
To summarize, the form that light took around 13.7 billion years ago to travel through the universe was in the form of the cosmic microwave background radiation, which exists as microwave photons with stretched wavelengths due to the expansion of the universe.