A radiation that is an after-effect of the Big Bang still around today. It was actually discovered accidentally by two Arno Penzias and Robert Wilson in 1965. They noticed a static that appeared to be coming from every direction at the same strength. Later scientists were able to trace out a Black Body curve for this background radiation, shown below.
Essentially, at very early point in the beginnings of the universe, everything cooled down enough to where atoms could form and this radiation was formed. The temperature to which it has cooled, estimated to be around -270 K, scientists believe to be consistent with a universe that started at a very high temperature and in a very dense state, the Big Bang. Therefore, this cosmic radiation is very strong evidence in favor of the Big Bang. 
Light travels at a velocity incredibly difficult for us to comprehend other then as just a very, very large number. To be exact, light travels at a constant velocity of 299,792,458 m/s within a vacuum. This is not the same for light traveling through other conditions. When light does travel through things other then a vacuum, there exists something called the refractive index. This index represents the amount that light is slowed. It represents this as the ration between the speed of light in vacuum and the speed at which the light travels within the medium, which is always greater then one since light can never travel faster then when it’s in a vacuum. What’s interesting is that this equation is also defined a different way. Instead of being the ration between two velocities it is defined as sin(i)/sin(r), where i is equal to the angle of incidence and r is the angle of refraction. Since this ration is always greater then 1, then it means that everything light passes through bends it in some way. This diagram from Encyclopedia Britannica represents a visual of the how light travels through a medium.