Usually, when a new particle is discovered or its existence hypothesised, it's on such a tiny scale that it's hard for us to imagine. But that might not be the case with dark matter, because researchers have found evidence to suggest that these mysterious, invisible particles could be about one-third the size of a human cell, and dense enough to almost create a mini black hole.
Though they reportedly make up five-sixths of all of the matter in the Universe, no one truly knows what dark matter is, how it works, or even what it could look like. Despite its mysterious nature, scientists hypothesise that dark matter has to exist in some form to account for the amount of mass needed for the Universe to exist and act in the way it does.
Knowing this, researchers from the University of Southern Denmark decided to investigate the size of these hypothetical hidden particles. According to the team, dark matter could weigh more than 10 billion billion (10^9) times more than a proton.
If this is true, a single dark matter particle could weigh about 1 microgram, which is about one-third the mass of a human cell (a typical human cell weighs about 3.5 micrograms), and right under the threshold for a particle to become a black hole.
The researchers came up with this number by creating a new model for a super-heavy particle they call the PIDM particle (Planckian Interacting Dark Matter). These supermassive particles belong to a class of particles known as 'weakly interacting massive particles', or WIMPS.
Before now, researchers have suggested that WIMPs were about 100 times the mass of a proton, Charles Q. Choi reports for LiveScience, but while the existence of WIMPS has been hypothesied for years, evidence of them is, well, extremely lacking, like everything else about dark matter. This leaves open the possibility that dark matter particles could be made of something significantly different, says Choi.
If the team from Denmark is right about the size of dark matter particles, it means dark matter is too large for researchers to recreate with particle accelerators. Instead, evidence of dark matter might exist in the Universe’s cosmic microwave background radiation, which is basically the light left around from the Big Bang.
In short, when the Big Bang happened 13.8 billion years ago, the Universe grew rapidly, a time period researchers call 'inflation'. The next stage on the Universe’s development chart is called reheating, which, among many things, created particles. It's here, during reheating, that supermassive dark matter particles might have first formed.
"However, for this model to work, the heat during reheating would have had to be significantly higher than what is typically assumed in Universal models," says Choi. "A hotter reheating would in turn leave a signature in the cosmic microwave background radiation that the next generation of cosmic microwave background experiments could detect."
Obviously, if we do eventually observe direct evidence of dark matter, it would solidify many hypotheses about how the Universe works and initially formed. However, before that happens, we need better tools, which University of Southern Denmark cosmologist, McCullen Sandora, says we should have within the next decade.
Until then, we can only speculate how dark matter works and how it fits into longstanding hypotheses and models.
You can view the team’s report in Physical Review Letters.