Dinosaurs vanished from Earth about 65 million years ago. Scientists are not sure why this happened, but they have a few theories. Some experts say that volcanoes destroyed the dinosaurs. Others think that their disappearance was caused by something falling from the sky, like a comet or asteroid. Some scientists think that it was a combination of both of these, or that the Earth’s surface shifted and the climate changed. Whatever happened, it stopped the dinosaurs from walking on the Earth, forever.
In the second sentence, what does “have a few theories” mean?