Something in the universe creates more mass than we can directly detect. We know it is there because of the gravitational effects on matter that we can detect; but we do not know what it is and where it came from.
We call this invisible mass 'dark matter', and physicists have just identified a particle that could be it.
The candidate is a recently discovered subatomic particle called the d hexaquark. And in the primordial darkness that followed the Big Bang, they could come together to create dark matter.
For nearly a century, dark matter has puzzled astronomers. For the first time, its influence was seen in the movements of the stars, which hinted that there was more mass around them than we could see.
We can now see the influence of dark matter in other dynamics – for example, with gravitational lensing, when light bends around massive objects such as galaxy clusters. And the rotation of galactic disks, which is too fast to be explained by its apparent mass.
Until now, it turned out that dark matter cannot be detected directly, since it does not absorb, emit or reflect electromagnetic radiation of any type. But its gravitational effect is strong – so strong that up to 85 percent of the matter in our universe can be dark matter.
However, scientists would love to understand the secret of dark matter. This is not only because they are very curious – figuring out what dark matter is can tell us a lot about how our universe formed and how it works.
If dark matter doesn't really exist, that would mean that something is wrong with the standard model of particle physics that we use to describe and understand the universe.
Several dark matter candidates have been put forward over the years, but we seem to be getting closer to finding an answer. Hexaquark d – more formally, d (2380) – enters the scene.
“The origin of dark matter in the universe is one of the biggest questions in science and still has no answer,” explained nuclear physicist Daniel Watts of the University of York in the UK.
'Our first calculations show that condensates d are a new possible candidate for dark matter. This new result is especially interesting, since it does not require concepts new to physics. '
Quarks are fundamental particles that usually combine in groups of three to form protons and neutrons. Collectively, these three-quark particles are called baryons, and most of the observed matter in the universe consists of them. You are baryonic. Like the sun. And planets and stardust.
When six quarks combine, it creates a type of particle called a dibaryon, or hexaquark. In fact, we haven't seen many of them at all. Hexaquark d, described in 2014, was the first non-trivial discovery.
Hexaquarks d are interesting because they are bosons, a type of particle obeying Bose-Einstein statistics, the basis for describing particle behavior. In this case, this means that the collection of hexaquarks d can form something called a Bose-Einstein condensate.
Also known as the fifth state of matter, these condensates form when a low-density boson gas cools to just above absolute zero. At this stage, the atoms in the gas move from their regular rocking to a completely stationary state – the minimum possible quantum state.
If in the early Universe such a gas of d hexaquarks was everywhere when it cooled down after the Big Bang, then, according to the team's modeling, it could have combined to form Bose-Einstein condensates. And these condensates could be what we now call dark matter.
Obviously, this is all highly theoretical, but the more dark matter candidates we find – and confirm or exclude – the closer we are to defining what dark matter is.
So there is still a lot of work to do here. The team plans to find d hexaquarks in space and study them. They also plan to do more work on hexaquarks in the lab.
“The next step for creating this new candidate for dark matter will be a deeper understanding of how hexaquarks interact – when they attract and when they repel each other,” said Mikhail Bashkanov, a physicist at York University.
“We're taking new measurements to create hexaquarks inside an atomic nucleus and see if their properties differ from when they are in free space.”
The study was published in the journal Physics G: Nuclear Physics and Particle Physics.
