LAUSR

As far back as the 1930s, astronomers began to suspect that every galaxy and every cluster of galaxies was surrounded by a massive haze of “dark matter”—an utterly invisible ectoplasm that held these photogenic yet fragile structures together with the stabilizing force of its gravity. These early observers were right: Not only does dark matter seem to be real, but it outweighs the visible universe—meaning stars, planets, nebulae, galaxies, and anything else made of ordinary atoms—by a factor of five (1). Yet the nature of dark matter remains one of the biggest mysteries in cosmology. One favored theory is that it’s made of weakly interacting massive particles (WIMPs) that barely interact with normal matter, except gravitationally. Also popular is a theory that dark matter is made of lighter but equally hypothetical particles called axions. But over the past half-decade or so, some researchers have become more open to an older idea: Dark matter consists of primordial black holes (PBHs) that emerged from the Big Bang. “If you’d asked me in the early days I would have said primordial black holes are interesting, but there’s maybe only a 10% chance they really exist,” says Bernard Carr, a physicist at Queen Mary University of London, UK. Carr is referring to the early 1970s, when he and his thesis advisor, the late Stephen Hawking, did the first detailed calculations for how the infant universe could have produced black holes, which are gravitational singularities in which the fabric of space and time curves in on itself so tightly that not even light can escape (2–5). But today, says Carr, who recently coauthored a recent review of PBHs as dark matter (6), “I would bet you at least 50% that they exist, maybe even more.” Carr cheerfully admits that a PBH pioneer like himself is hardly a neutral observer on this subject—and he is quick to acknowledge that PBH formation requires a fine-tuning of cosmological parameters that many researchers find implausible. Still, his cautious optimism can also be heard from other cosmologists. They point to three primary reasons for the shift: first, attempts to find WIMPs or the other hypothesized dark matter particles have come up empty; second, results from gravitational wave experiments looking for ripples in spacetime are surprisingly consistent with the PBH idea. And third, the next decade or two could bring observational evidence that either confirms the existence of PBH dark matter or rules it out.