וַיַּבְדֵּ֣ל אֱלֹהִ֔ים בֵּ֥ין הָאֹ֖ור וּבֵ֥ין הַחֹֽשֶׁךְ׃
And God separated the light from the darkness
Some things just cannot exist together. After filling the universe with light, God reintroduced darkness. Light destroys darkness, but it's full effect can be toxic. Since the earth was to be the home to plants and animals that ate plants (or at least the fruit and seeds), Light had to be dosed properly. Darkness had to exist for life to thrive.
Essentially, visible light is a small part of a wide spectrum of massless particles, or virtual particles existing in vibrations. Light reveals the substance hidden in a vast desert of "empty" space. These vibrations can be deadly at frequencies on higher than what we call violet, a dark blue color. On the other end, long invisible waves of light provide life sustaining heat beyond the red we see. Other benefits from these longer waves include radio waves that have been harnessed to carry information long distances.
But why introduce darkness? Too much of a good thing can be harmful. Even safe white light can tire out ones eyes. Plants especially need time to process the energy they absorb. It is in the darkness that many lifeforms rest, heal and grow strong.
The phrase וַיַּבְדֵּ֣ל ... בֵּ֥ין means "he distinguishes between". The verb is בָּדַל, meaning "separate, distinguish; divide". God begins creation by setting limits and definitions. Light and darkness are opposite from each other. It turns out that heaven and earth manage on different scales by somewhat different rules.
Light and other tiny particles are affected by heavenly large scale principles such as gravity, but when they are "by themselves" they cooperate by rules we can only discover by trace evidence left behind.