When the universe came into existence some 13 billion years go it was, for a short time, composed of pure energy. So light came first. It's safe to say that a tiny area of space that was 1000 trillion trillion degrees celcius and composed almost entirely of photons didn't have any shadows in it. And for the people arguing for dark being first, there wasn't a big dark chasm before the universe was created, there was nothing at all.
I guess darkness came first whether you go by the traditional Christian ideology (you know, there was darkness and nothing and the Lord said "Let there be light"). Or whether you go by the scientific route which many accept as the big bang theory, in which case there was nothing before the big bang until it happened. And if there is nothing then then is no source of light, and thus all would have been dark. Darkness falls where there is no energy sufficient enough to produce it, and there's a lot. Sure there are a lot of stars, but there's even more empty spaces of cold, dark voids in between
Light and dark cannot exist outside of time and space, which burst into existence simultaneously in this universe. However, in the very first fraction of a picosecond, it is probably okay to believe that there was a saturation of light.
Neither. Nothing existed to start off with. Although something must have existed for it to start, or is it because humans think there must be something to start? When in fact thats not the fact and that we cannot comprehend it. Because the brain works on logic. So, in the mind something has to be there before something is made.