Listen, a comet hasnβt hit earth in 66 million years, and earths atmosphere will most likely stop the comet. Most meteors are small and will burn up when they pass through the Earth's atmosphere. π€
You don't have anyway to tell what happened or when.
A comet could have hit the earth 200 years ago and you'd have no way of knowing. A comet could hit the earth in a week, and you wouldn't know about it until after it happened.
The list of things we don't know about the universe is quite extensive...