There are many theories about why the dinosaurs died out seemingly all at once nearly 65 million years ago. The most popular idea of the mass dinosaur extinction is the "Meteor Theory."
Scientists believe that during the time of the dinosaurs, a giant meteor collided with the Earth. The tremendous impact blasted dust into the atmosphere, blocking out sunlight, and plunging Earth into the blackness and cold of a sudden, short-duration "impact winter."
The dinosaurs could not adapt fast enough to the sudden winter and as a result, died out completely. Scientists also refer to this catastrophe as the "Great Extinction" or the "K-T Extinction." K-T is the label that scientists give to the geological layer which corresponds to the late Cretaceous Period.