Because a Fermi-like signal requires dark matter to turn into photons, which are a form of ordinary matter, an observable interaction will arise only if there is a particle that is charged under both the usual electromagnetic force and the dark one—the analogue of the person who both watches Fox News and listens to NPR or is signed in to both Facebook and Google+.
A terrible analogy is a beautiful thing. It is almost a pity that this book is doomed to be quickly forgotten, as having people read it in 20 years and ask "What is Google+?" would be somewhat wonderful.
