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WHAT鈥橲 the matter with dark matter? Its name, for one thing.

Dark matter is so-called because of the idea that it is like being in a room without a light on. But actually, we know the universe is filled with light even with lots of dark matter in it. We see evidence of this very often 鈥� directly from the sun during the day and reflecting off the moon at night. On a clear night we can see stars too. With sensitive instruments, we can also detect the cosmic microwave background radiation that pervades all of space-time.

The universe isn鈥檛 like a room without a light on. It is much more like a giant room with billions of lights spread out all over the place.

Light goes right through dark matter 鈥� it is transparent. So transparent matter or clear matter would be a better name. However, we are in the dark about what exactly dark matter is.

We know it interacts with gravity just like the matter we can see (like people and planets) and we know it moves slowly. What we don鈥檛 know is how to write down an equation that describes its quantum nature and therefore its relationship (or lack thereof) with the standard model of particle physics. Knowing so little about dark matter is a fairly strange predicament because we have been able to work out that it comprises most of the matter in the universe. Normal matter only makes up about 20 per cent.

The fact that we know dark matter exists is a lovely detective story, one I have touched on in a previous column (18 May 2019, p26). The first compelling evidence for dark matter came from Vera Rubin using a device made by Kent Ford. She measured the speeds of stars as they rotated around the centres of their home galaxies. Using these speeds, she calculated how massive the stars are. Adding up all of those masses, she was able to get a total mass for the galaxy. This was greater than the mass calculated using the amount of light the stars radiate. This discrepancy indicated the presence of matter that we couldn鈥檛 see, something which had been hypothesised for a century.

There is now extensive evidence from other observations that there is a lot of this subluminal matter, as Nobel Laureate Jim Peebles calls it in his new book, Cosmology鈥檚 Century: An inside history of our modern understanding of the universe.

These observations include strong gravitational lensing, in which dark matter between us and a distant galaxy is so massive that it bends space-time and makes it act like a funhouse mirror, warping the galaxy鈥檚 light. It is incredibly difficult to explain such observations with alternative models. Dark matter, despite the mystery over what it actually is, is the simplest explanation we have.

鈥淜nowing so little about dark matter is a strange predicament because we have worked out that it comprises most of the matter in the universe鈥�

It is easy to think this mystery is merely one of fundamentals: what particle is it made of? But not knowing this has a domino effect on other areas of astrophysics too.

For example, my recent work has focused heavily on trying to understand how large galaxies form, how their satellite galaxies form and the relationship of central galaxies and their satellites to dark matter halos 鈥� giant collections of dark matter 鈥� which envelop them. This turns out to be difficult to understand, partly because we can鈥檛 see dark matter, but also because we don鈥檛 know what details to put into our computers to help us simulate how galaxies and their halos form.

In my column of May 2019, I also wrote that my preferred dark matter candidate is the axion, a hypothetical particle that helps solve a problem in the standard model of particle physics, and I promise I haven鈥檛 abandoned it.

Instead, I have thrown myself into the question of how the relationship between galaxies and their halos evolve if dark matter is made out of axions. Of particular interest is the unusual behaviour that axions seem to display.

There is good reason to believe that, unlike many other dark matter candidates, axions can go into exotic quantum states known as Bose-Einstein condensates. In this state, all of the particles act as one, creating a macroscopic quantum wave. This trait of axions would lead to different galactic centres than ones expected from other dark matter candidates.

Questions about axion Bose-Einstein condensates remain. For example, how the condensate state forms depends on what forces are at work. In a paper I am working on, my colleagues and I calculated the timescale for condensate formation depending on such factors.

We found that if we ignore gravity, it takes 10 million times longer! We now feel confident that gravity plays an important role in getting axions into this condensate state, which will help us model the evolution of galaxy halos made out of these particles. These models can be compared with data, and if they match, this will mean that studying what dark matter does can provide a hint about what dark matter is.