Black holes formed naturally at the present epoch of the universe have a minimum mass of between two and three solar masses, but much smaller ones might have been formed in the very early universe. Study of these has a long history going back to seminal papers by Zel'dovich, Novikov, Hawking and Carr more than forty years ago. Recently, they have attracted renewed attention in connection with their possible role as dark matter. Indeed, they are widely seen as the last hope for explaining the dark matter within the standard model of particle physics. A recent paper claiming to have ruled out this explanation has been strongly criticised. So what is the situation? Are these primordial black holes genuinely interesting?
