Haehnelt points to evidence from recent studies of the cosmic microwave background (CMB). This radiation, sometimes called “the echo of the big bang” has been travelling unaltered through space since the universe was just 400,000 years old. At that moment the universe cooled through a critical point, letting CMB radiation travel freely for the first time – as though a cosmic fog had lifted. But new evidence shows that 10 to 15 percent of this radiation has been scattered since then. This indicates a re-warming of the universe which nobody had expected.

Haehnelt explains that this could indicate an era in which small black holes were commonplace. “Matter accreting around a black hole heats up,” he explains, “and this heating could be a sign that small black holes were widespread in the Universe at that time.”

If small black holes merged to form the supermassive variety found at the centres of galaxies, there could be telltale evidence. Such a merger begins with two black holes going into orbit around each other, spiralling ever closer together. In the cataclysmic blast of energy when they finally merge, any asymmetry can send the resulting black hole flying off into space. “If this happened,” says Haehnelt, “we might find the occasional galaxy with its central supermassive black hole missing.”