Could we have been looking for dark matter in completely the wrong place – or rather, at the wrong mass?
A team of researchers attempting to understand some unexplained chemistry at the centre of our Galaxy, the Milky Way, think we might be.
Dark matter is an invisible substance that makes up around 85% of the Universe’s mass.
While it can’t be seen directly, its impact can be seen in the rotation of galaxies, light bending around massive clusters, and even in the echo of the Big Bang, the cosmic microwave background.
The current leading theory is that dark matter is a group of particles known as ‘Weakly Interacting Massive Particles’, or WIMPs. These only interact with so-called ordinary matter via gravity.
While individual WIMPs can move through ordinary matter without affecting it, large clouds of them have enough mass to shape the Universe.
A new type of ‘light’ dark matter
A study looking at the centre of our Milky Way Galaxy, a region called the Central Molecular Zone, may provide a different option.
“At the centre of our galaxy sit huge clouds of positively charged hydrogen – a mystery to scientists for decades because normally the gas is neutral,” says Shyam Balaji from King’s College London who led the study.
This means the gas has been ionised – the negatively charged electron has been knocked out of the atom. This requires a lot of energy, so the question is where does that energy come from?
Balaji’s team propose the culprit could be a new form of dark matter particle. These crash into each other, creating showers of charged particles that then ionise the hydrogen gas.
“The energy signatures radiating from this part of our Galaxy suggest that there is a constant, roiling source of energy [ionising the gas], and our data says it might come from a much lighter form of dark matter than current models consider,” says Balaji.
The search for dark matter
Though observations of the Universe led to the theory of dark matter, the experiments looking for it are almost all based here on Earth.
These look for signs of the rare occasions when a dark matter does interact with normal matter.
The huge experiments are usually buried deep underground – such as at the bottom of an old Yorkshire mineshaft – where they are shielded from other forms of interference.
“The search for dark matter is one of fundamental science’s most important objectives, but a lot of experiments are based on Earth, waiting with hands outstretched for the dark matter to come to them,” says Balaji.
“By using gas at the CMZ for a different kind of observation, we can get straight to the source. By peering into the centre of our Milky Way, the hydrogen gas in the CMZ is suggesting that we may be closer to identifying evidence on the possible nature of dark matter.”
