How would you know it’s light if there was no dark to contrast it ? Light only exist because dark surrounds it, and dark only exists because light surrounds it. One cannot exists without the other.
If I show you a black circle on a white paper you would point at the black and say “this is the thing that is drawn on the paper”. If I were to show you a white circle on a black paper, you would point at the white instead with the same statement. If I showed you an all white paper and told you there is a white circle on it, you would tell me I’m an idiot and that there is nothing there. Contrast is why something exists.
I understand you want to counter this statement with physics, and we could have a rich debate about what we know of the universe and how light and it’s absence exists in it. But I think you misunderstood what we are talking about.
This statement is about philosophy, light and dark are metaphorical here. We could just as well say “up cannot exists without down”, or “day cannot exists without night”. The next step to this philosophical thinking is to realise that since one cannot exists without the other, therefore they are the same thing.