Billions? According to theists the universe is only 6000 years old.
Actually, most believers I know will admit that they don't really know such things. We also don't really care how long its been. We also don't deny that there were dinosaurs or think as some of those funny meme's state, that Jesus rode a dinosaur.
We just believe in the Christ. That God, like a human parent, doesn't tell His children everything because we wouldn't understand it. Does an infant understand how a car works and would anyone try to tell a baby that is too young, too mentally undeveloped to begin to fathom something so "simple" to us but complex to them?
No.