According to Mrs. Whatsit, why is Earth such a troubled planet?

The earth is shadowed by darkness.

In the crystal ball, the children can see their own planet, but it is clouded by the Darkness they had seen earlier. Meg asks if it had just shown up, and Mrs. Whatsit tells her that it has actually been there for a long time. This explains why Planet Earth is so deeply troubled. They tell her that this Dark Thing is Evil, the “Power of Darkness,” but that their planet had been instrumental in fighting the Darkness through time.