Dark matter is a mysterious form of matter that is not visible to us, and is thought to make up about 27 percent of matter in the universe, according to NASA. Astrophysicists have long thought that this theoretical matter cannot cool itself down by releasing some of its kinetic energy—and thereby heat—the way other forms of matter can.
That theory makes sense because if dark matter could cool itself, then loose, random dark matter particles could coalesce. A dense enough clump of these particles could begin building itself into a compact object—the same process by which other kinds of matter gradually build into galaxies, and the celestial bodies inside them such as stars and planets. The more scientists understand about dark matter, the more they can understand about galaxy and planet formation.
“We assume dark matter doesn’t cool, because if you take a beautiful spiral-disk galaxy like the Milky Way and look at what dark matter appears to be doing, it’s all in this big kind of fluffy halo, like a cloud,” Matthew Buckley, a physicist at Rutgers University, told Newsweek, referring to what are known as galactic halos. “If it had cooled, that halo would have collapsed.”
Buckley and a colleague from Rutgers University have created a new theoretical model in which dark matter is able to cool after all. According to their research, if most dark matter cooled, then it would still reside inside its galactic halos. Some of it, though, could be scattered in small pockets throughout the galaxy. A paper describing the research was published earlier this month in the scientific journal Physical Review Letters.
Dark matter refers to matter that we can't see, but the matter that we can see—the kind that the Earth is made from—is called baryonic matter, and it's composed of familiar charged particles like protons and neutrons. It makes up about five percent of matter in the universe, according to NASA. Baryonic matter can cool, and thus form objects like the Earth, because it contains charged particles—the interactions between which are a prerequisite for creating larger structures.
Dark matter isn't thought to contain charges the way baryonic matter does; its particles would only be capable of weak interactions, Buckley said. Plus, if dark matter radiated away from its galactic halos—as it would if it cooled—it would leave those halos structurally weakened, and vulnerable to collapse.
This reasoning is why astrophysicists have believed that dark matter cannot cool. But without concrete evidence, it's possible that the reasoning is wrong. What if dark matter particles did have a something like a charge, and could be released without collapsing their halos, and we just hadn't yet seen any of it?
In the new theoretical model from Buckley and his team, dark matter contained two distinct varieties of charged particles—you can think of them as the kind of mirror-image counterparts of protons and electrons. It showed that such particles could, in fact, radiate energy, and do so without collapsing their galactic halo, by accounting for crucial variation in the dark matter density from one halo to the next. Some halos contain so much dark matter that the particles can never release and cool; it really is just stuck there. But in halos under a certain size, some dark matter might be able to cool and begin forming compact objects after all.
“The biggest possible clump that we found could still cool efficiently is a Milky Way-size galaxy," Buckley said. "The Milky Way hasn’t collapsed of course, but there might be smaller clumps within it that did.”
What exactly those objects might look like, he doesn't know. Size-wise, they could be anything between supermassive stars all the way up to dwarf galaxies.
"With dark matter, there's a lot of debate and [different] theories," Buckley said. "My motivation for this idea is that I’d like to go and prove that it’s wrong.”