A number of different theories have been proposed to answer this question. The most commonly accepted explanation is that a meteor wiped them out.
The asteroid that wiped out the dinosaurs(and many species alongside them) is estimated to be about 6 miles(10 km) across. That's the size of Mount Everest!
Evidence For An AsteroidEdit
There are a number of pieces of evidence in support of the asteroid theory. A large crater found off the coast of Mexico matches up with the needed size and age of the asteroid. The crater is 65 million years old, the same time the dinosaurs went extinct. Also, across the globe is a "ring" of iridium that dates back to 65 million years ago. Iridium is rare on Earth, but common in asteroids and meteors.
Other TheoriesEditSome other theories that have been proposed include, but are not limited to...