In 2004 a U.F.O. approached Earth. It began to release giant clouds of alien spores into the upper atmosphere that remained there. The spores eventually multiplied until they covered the entire Earth and blocked out the sun, casting the inhabitants of Earth into darkness. This would eventually be called the "Twilight" by those who would survive.
The spores, after multiplying many times, began falling to Earth. They began to smother everything and everyone. Most life was wiped out during this period which came to be know as "Nightfall".
The governments did nothing, hoping to not provoke the alien-like substance. Unfortunately this was their downfall as the end came all too quickly. Numerous humans did survive however, living out the months underground until the spores seemed to dissolve and decompose.