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ALL of the students in my astrophysics class last semester had to give a final presentation. In one, a student showed everyone an image of our galaxy, the Milky Way, as viewed from Earth. The student then showed another image of a spiral galaxy, suggesting that it reflected our own. Because it was a class focused on stars 鈥� the students are taking on galaxies this semester 鈥� a question came from the audience: how can we get a picture of the whole Milky Way if we are inside it? We can鈥檛.

In this sense, astrophysics is like a Midsomer mystery. There is a lot happening, and some of it is relevant to questions you might have and some of it is a distraction. Also, just as with all of the murders in England鈥檚 fictional most deadly county, to understand what happened, you can鈥檛 simply reconstruct the events and watch them happen.

Instead, it is necessary to piece things together using the information available. This is in stark contrast to experimental sciences, where it is possible to do an experiment multiple times to make sure you know what happened and where you can control the circumstances of the physical phenomena you are studying.

In astrophysics, we have no control over what we see. We can鈥檛 even leave our own galaxy to get a good look at it. But we can deduce some things. First, we know from looking around that galaxies take on only a few different shapes. Almost all galaxies we observe are either spiral, elliptical or something between the two, known as lenticular. There are also abnormal galaxies, but these are rarer.

To figure out what kind of galaxy we live in and what it looks like, we use circumstantial evidence. When we see pictures of the Milky Way, we are seeing only part of it, from inside it but near the edge. If you have ever had the chance to go somewhere that recognises the sanctity of dark skies, on a clear night you would have seen a band of bright dots that seem closer together as you get to the centre of the line.

We see the Milky Way like this not because our galaxy is a line but because we are looking at it from the side, rather than the top. This is a lot like looking at a fancy plate edge on: it looks almost like a line and you can only see the textures that the potter put on the side.

You can鈥檛 really see whatever colourful flourishes they put on the surface of the plate. However, we can measure how fast some of those stars are moving, and the speeds in the Milk Way 鈥� assuming the presence of dark matter 鈥� are consistent with a spiral galaxy.

鈥淔iguring out galaxy structure is one of the most exciting questions if you wonder how the universe works鈥�

Instead of actual pictures of the Milky Way, what we have are images of galaxies that we think are very similar to ours. For example, we spend a lot of time studying the Andromeda galaxy because it is the large spiral galaxy closest to us, and its proximity means that we can get good images of it. The question, of course, is how do we know it is at all like ours?

As I said, it is a detective-like effort. It would be easy for me to say, as a theorist who thinks about fundamental physics, that understanding the structure of galaxies is somewhat simpler than trying to figure out dark matter 鈥� the missing matter problem 鈥� and dark energy, the cosmic acceleration problem.

Although we are used to thinking of galaxies as bright collections of stars and dust, they are actually mostly collections of dark matter with a smattering of stars and dust inside. This is because, as I have mentioned in previous columns, dark matter makes up the majority of the matter in the universe. It is also the case that the phenomenon of cosmic acceleration, whereby the expansion of space-time is accelerating, changes the cosmological timeline for how galaxies form and cluster together.

In reality, figuring out galaxy structure, and how it is affected by dark matter, is one of the most exciting questions to face those of us who wonder about how the universe works on large scales.

There are still many questions about the Milky Way that remain unanswered. For example, we know it has small satellite galaxies: almost 60 of them. Two have names that you may have heard before because they are visible with the naked eye: the Large Magellanic Cloud and the Small Magellanic Cloud. You might wonder why the number is about 60. So do astrophysicists.

In fact, I have a PhD student doing work that seeks to address exactly this question. We don鈥檛 understand the formation history of the Milky Way and its dwarf galaxy satellites, collectively known as the Milky Way subgroup. In other words, you don鈥檛 have to go far in the universe before you run into some of its biggest cosmological questions.