The second Chabad Rebbe, Rabbi Dovber of Lubavitch, explained the tremendous darkness of the exile in the following manner:
Before daybreak the darkness intensifies, as the light of morning is very close. So too, is with the tremendous darkness of this exile. Its many terrific hardships are preparation for the great light of Moshiach; a light that is even greater than the one revealed at the giving of the Torah.
Such is the measure; after a sickness comes healing and after a tragedy comes salvation and consolation.
 Imrei Bina 5b