Hazardous comets and asteroids are tracked by various space agencies, but British-based astronomers say there are many dangerous ones still going undetected, New Scientist magazine reported.
Previous research suggests comets nudge in the Earth's direction when the solar system periodically passes through the galactic plane.
These comets appear to coincide with the dates of ancient impact craters found on Earth — suggesting they were caused by comets and not asteroids.
"There is a case to be made that dark, dormant comets are a significant but largely unseen hazard," said Bill Napier at Cardiff University.
Napier and David Asher, at Armagh Observatory, say dark comets may still be in the solar system, and could come near Earth.
Dark comets happen when a comet's reflective water ice evaporates away, and leaves behind an organic crust.
Since the water's gone, they don't reflect light, making it hard for astronomers to see them until they're right on top of us.