If contrast were to cease expansion would. And expansion is necessary for eternity.
This dualism begins with the symbol of the sun, and with the phenomena of light. The sun gives warmth and the sun illuminates. But the sun can also scorch and blind. The sun is attractive but it can not be gazed upon directly. The sun is magnificent and powerful yet its heat and light are finite. A match or a candle, a torch or a flame, burns brightly and attractively but burns to burn out. Yes, there are many properties of light that we might not be too aware of. To the ancients the sun had at least three faces or aspects. There was the sun in the morning, the sun at midday,and the sun at evening. Then there was the southern sun, and the black sun of the galactic center. Even science knows of the billions of suns contained in the universe.
Beauty has a dark side. Sexuality has one. Mercy has a dark side and so does love. Anger has a good side and hate also. This is the way that the ancients saw life. For gods of light there were the gods of darkness. For gods of the overworld there were the gods of the underworld. However, it would be more correct to say that the gods of light had a dark side. It was only later, after the ruination of the original stellar gnosis, that mythmongers and priests began to push the notion that the two aspects of a single archetype were distinct entities, separate, divided, adversarial, and irreconcilable. Nothing of this kind was taught in the mystery schools of old. It is a concoction of the perverse and it has caused untold folly to occur. Aton, like Jehovah, has his two sides; the benign and the wrathful.