Light equals darkness.
Darkness equals light.
By definition, they each owe their existence to the other, and in equal proportions.

If light is a metaphor for knowledge, and dark for ignorance, our increased knowledge only creates more questions.

But I think Einstein simply meant there is no end to learning/discovery, that it is a journey with no destination possible. So we shouldn't be so darn cocky over the little bit we think we do know.

Or I could be wrong.