Light has come whenever God has revealed truth to mankind, coming most completely in the person of Jesus Christ. This is one of the points John makes clear in the prologue to his gospel. Katalamban has a primary meaning of seizing with force or surprise. The point of noticing this is that in the Scripture authored by John, as well as in other Scripture, there is a constant conflict between light and darkness. In the physical world in which we live darkness is passive.
Darkness exists wherever there is an absence of light.
Wherever light shines, darkness is automatically eliminated. But this is not so with spiritual darkness. Spiritual darkness is not passive, but active, and it is active on two levels. On one level, darkness is promoted by spiritual forces under the leadership of Satan. The world may sometimes doubt whether Satan exists, but neither Scripture nor Jesus express any such doubt.