Astronomer David Asher (from Armagh University) has agreed with Bill Napier and Janaki Wickramasinghe (Cardiff University) that “dark comets” are real and dangerous.
The following quotes are from a paper by Napier and Asher published in Astronomy & Geophysics.
http://star.arm.ac.uk/preprints/2009/539.pdf

We know that about one bright comet (of absolute magnitude as bright as 7, comparable to Halley’s Comet) arrives in the visibility zone (perihelion q<5AU, say) each year from the Oort cloud. It seems to be securely established that ~1–2% of these are captured into Halleytype (HT) orbits. The dynamical lifetime of a body in such an orbit can be estimated, from which the expected number of HT comets is perhaps ~3000. The actual number of active HT comets is ~25. This discrepancy of at least two powers of 10 in the expected impact rate from comets as deduced from this theoretical argument on the one hand, and observations on the other, is …