Over the past few years, black holes have taken a compelling hold on scientific and popular discourse about cosmology and the nature of the universe. For many people these bizarre objects, which pack matter into a small space so densely that absolutely nothing can escape their gravitational pull, are the most fascinating objects in space.

Although some physicists started to predict their existence a century ago, as a consequence of Einstein’s theory of relativity, it took many decades of astronomical observations to move beyond the mathematics and produce compelling evidence that the universe really contains huge numbers of black holes. Even 20 years ago, many scientists questioned their reality; today almost everyone is convinced not only that they exist but that they play an essential role in the formation of stars and galaxies — indeed in the overall distribution of matter through space.

Now scientific speculation is building around objects that are the hypothetical inverse of black holes: white holes, which matter cannot enter but will eventually leave. Speculator-in-chief is Carlo Rovelli, the Italian theoretical physicist who is also a maestro of imaginative science writing.