Light dispels darkness. The two of them cannot exist together at the same time and in the same place.  If one comes, the other has to go; but where light is abundant, darkness is scarce.

The people of Galilee walked in darkness because they were a people who also had the reputation that they could never be trusted.  In addition, the people of Galilee lived the furthest away from Jerusalem, the center for all religious activities.  So there was no true worship of God practiced by those in Galilee.  These people had become spiritually weak and careless and grossly superstitious even turned to cultic worship practices.

So when Jesus' coming into Galilee, where the darkness had been the greatest, there He brought the first light of the gospel.  His ministry of calling to repentance and faith was in itself already a great light in Galilee.  The Galileans rejoiced when God showed them a great light, and that great light went into their darkness and preached the word of life and light to them. 

For them, the light had come because Jesus was the light of life!