杏吧原创

IF YOU ever visit a nature reserve that has worked with the International Dark-Sky Association to preserve its view of the stars, then on a clear night you can see the Milky Way. Although we are certain our home galaxy is a spiral, to the naked, uninformed eye, it looks like a curved strip made up of many little dots of light. There are so many stars that they can seem uncountable.

Although those little lights seem mystical, they can also feel familiar, like a distant street lamp. We now know that they are far away suns, made up mostly of hydrogen with some helium and other heavier elements in the mix. Stars are made of the same stuff as us. That might make you think that most of the stuff in our galaxy comprises the same kind of matter as we find in our bodies, but actually it doesn鈥檛.

As I鈥檝e discussed in previous columns, stars rotate around the centre of galaxies faster than we would expect because the galaxies seem to be more massive than the number of stars indicates. There appears to be more matter in galaxies than the stuff we can see. This has often been called the missing matter problem, with the missing stuff being labelled 鈥渄ark matter鈥�.

In reality, it seems that most of the matter in our galaxy is this unfortunately named dark matter. As I have written before, my issue with the name is that dark matter 鈥� if it is real 鈥� isn鈥檛 really dark at all. It is transparent. Or invisible, if you prefer.

So, it turns out that when we look at the Milky Way at night from a dark-skies nature reserve, we aren鈥檛 seeing most of our galaxy. We are just seeing the luminous, visible part.

At this point, dark matter is just hypothetical. We have never seen it because it doesn鈥檛 radiate light. We only know of it because we can infer its existence from observations. For example, sometimes we use our telescopes to look at galaxies that appear to have strange features encircling them. This is a result of something known as gravitational lensing, and it is caused by massive objects bending the space-time between us and a distant galaxy, leading to distortion of the image of that galaxy. It is hard to explain gravitational lensing in some cases without the existence of dark matter.

This isn鈥檛 even our strongest evidence for dark matter. What is? Observations of the cosmic microwave background radiation (CMB), a phenomenon I discussed in last month鈥檚 column.

鈥淢ost of the matter in the universe is invisible 鈥� visible matter like stars, and us, is unusual鈥�

Although photons from the CMB originated in the early universe, their journey through space-time has been affected by its contents, including dark matter, which has left an identifiable imprint on patterns in the CMB. The CMB is just one example in a litany of data that points to one conclusion: not only does dark matter exist, but it also makes up most of the matter in the universe.

In other words, it turns out that when we look at the stars, we aren鈥檛 seeing what is typical in the universe. We are seeing what is atypical about it. Most of the matter in the universe is invisible to us 鈥� and visible matter like stars, and us, are what is unusual, accounting for only about 20 per cent of the matter in the universe, with dark matter making up the other 80 per cent.

The implications of this for our galaxy and others are quite literally massive. We have deduced from looking at the way that stars move that each galaxy lives in a dark matter bubble that we call its dark matter halo.

I posted this bit about dark matter haloes on social media recently, and a follower asked me the astute question: 鈥淒oes the number of haloes match the number of galaxies?鈥� In other words, do galaxy numbers, and the properties of galaxies, correspond directly to halo numbers and halo properties?

This is what is known in the cosmology community as the galaxy-halo question, and it is an exciting topic of research because we don鈥檛 know the answer. My most recent research paper, a preprint that I worked on with my student Noah Glennon, is helping us make progress on this question in relation to my favourite dark matter candidate, the axion 鈥� a hypothetical elementary particle.

In the paper, we explored how dark matter haloes made of axions can be modelled with simulations using computer code that Noah helped build. For our follow-up efforts, we are lucky enough to be working with world-leading galaxy-halo expert Risa Wechsler and her group at Stanford University and the SLAC National Accelerator Laboratory in Palo Alto, California.

In the midst of a difficult year, I鈥檝e often been asked how I can possibly find meaning in the work that I do. There are certainly times when I have doubts. But, for now at least, the night sky is still a busy place, and it turns out that there is more to it than we thought.