Without dark matter there would be no galaxies, stars or planets. But physicists are yet to find direct proof of its existence. So where is it?by Philip Ball / May 8, 2019 / Leave a comment
Most of the Universe is missing and decades of searching have so far elicited no sign of it. For some scientists this is an embarrassment. For others it is a clue that might eventually push physics towards the next frontier of understanding. Either way, it is an odd situation.
Science has hunted in vain for the missing material. Its existence has never been detected directly, only inferred from hints. Yet if the rest of what we know about the way the cosmos is structured is right, it must be about five times more abundant than all the matter we can see in the Universe.
“Dark matter” is truly ghostly stuff. It is hidden far more profoundly than black holes, about which there was much excitement in April when a beautiful image of one was produced, showing a yellow-orange blob with a black void in the middle. Although, virtually by definition, we won’t ever truly “see” light-swallowing black holes, we can see their effects on the surrounding matter and space. More to the point, we are pretty sure we know what they are made from: ordinary matter, the stuff of stars.
Dark matter is something else entirely. And yet most scientists are agreed that this elusive material must exist: without it, they find it hard to see how we could be here at all. The gravitational pull that it exerts—the only impression it leaves on the visible Universe—is an essential ingredient for the formation of galaxies, stars and indeed planets like our own.
This is science’s most necessary fiction: a stark reminder that there are some important pieces missing from the puzzle of how the laws of nature work. Its elusiveness raises fundamental questions about how we should conduct scientific inquiry. Must we, as Isaac Newton thought, only believe in what we can demonstrably prove exists? Or do scientists need the freedom to imagine what cannot yet be observed—and may never be—to pave over gaps in current understanding so that we can still walk the path?
Science has engaged in many potential wild-goose chases before. Sometimes, as with the pursuit of the particle called the Higgs boson, which was finally found in 2012, decades of faith and searching paid off in the end. But in other cases, a long and fruitless search may…