The original observation about galaxy rotation curves never meant much to me. I didn't know enough astrophysics to have an intuitive appreciation of the significance of the disagreement between the theory and the measurements, and I felt that the universe was a sufficiently large and mysterious place that some unexpected observations — particularly at the scale of galaxies — were inevitable. It seemed arrogant to me to expect that vast, complex cosmic structures should perfectly obey the predictions of a classical theory that had been developed centuries before it was known that galaxies existed; before their scale, composition and distance from us were understood, and before it was known that we were inside one. I thought — and still think — that the universe is probably full of undiscovered phenomena, and that the discrepancy would gradually go away as more was learnt about the types of matter already known to physics; how much of it exists, how it behaves on large scales and how it is distributed.

I think I felt that the most likely explanation for the observations was what are now referred to as MACHOs (Massive Compact Halo Objects); massive but dark objects, like black holes, dust clouds etc., and I presumed there must be a lot more of them than thought. The assumption that the anomaly must be due entirely to a new form of matter seemed to me like jumping to conclusions; an unjustified leap.

One thing that began to change my mind was learning more about particle physics. I found it interesting that certain well-understood forms of matter were known to be oblivious to some of the fundamental forces of nature. Hundreds of billions of neutrinos pass through our bodies every second almost completely without trace because they do not feel either the electromagnetic force or the strong nuclear force. I did know about neutrinos when I first formed my views about dark matter, but I initially didn't connect the two.

As I thought more about the ability of some kinds of matter to ignore certain forces, I wondered whether there might be as-yet undiscovered forms of matter even more reluctant to interact than neutrinos are (neutrinos do at least interact via the weak nuclear force, hence how they have been detected). Given that neutrinos don't feel the electromagnetic interaction that dominates our visible world, perhaps there could be undiscovered forces that the matter we are made of is insensitive to as well. This led me to wonder whether there might be a whole zoo of 'shy' particles, perhaps similar to the normal luminous matter we are familiar with, but with their own complex interactions, structures and forces; existing around us, in the same spacetime we inhabit, but unseen. The idea of mutually invisible and non-interacting worlds existing side-by-side was fascinating to me; this is usually the province of speculative fiction, but perhaps there was some analogue to it in the real universe.

I found that this 'dark sector' physics was an active area of research for credible scientists. My understanding is that the latest work tends to indicate that this kind of self-interacting matter can't be the dominant component of dark matter, but the possibility of a role for unseen structure like this remains open, and the idea was so interesting to me that it prompted me to reconsider my views about dark matter.

I knew that before its detection was first confirmed, antimatter had been hypothesised to exist based on consideration of mathematical symmetries in physics. Working through the consequences of the symmetries of nature turned out to be a very productive method for discovering new particles, and the approach culminated in the theory of supersymmetry. In the early to mid 2000s, supersymmetry was well-established among leading particle physicists, and was regarded as the likey future of the field (along with the Higgs boson, it was a central motivation for the construction of the Large Hadron Collider). Supersymmetry predicted that many more particles and forces should exist than predicted by the standard model of paticle physics, and at some point I learned that it was a feature of supersymmetric models that the lightest particle they predict has the properties needed to explain the abundance of dark matter in the universe. This is known as the 'WIMP Miracle' (WIMP standing for Weakly Interacting Massive Particle). That the leading particle physics theory of the time was predicting the existence of a particle that just happened to have the properties needed to explain the observations that had first led dark matter to be postulated struck me as unlikely to be a coincidence.

I understand that supersymmetry has since fallen out of favour to some extent, partly because of disappointment that the LHC hasn't yet found the evidence for it that the particle physics community was hoping for, but I think it would be fair to say that it remains the most credible framework for future research at the moment.

At around the time I first learned of the WIMP Miracle I was also coming to understand more about cosmological inflation. In addition to its predictions being borne-out by measurements of the cosmic microwave background radiation (the Boomerang and WMAP experiments had recently pubished their results), inflation had several other properties that appealed to me, and the theory also happened to predict a mass-energy density for the universe that corresponded very well with the observationally derived abundance of dark matter.

Since then, more recent work to map the distribution of mass in the universe using gravitational lensing; computer simulations that strongly suggest dark matter is required to explain the observed large-scale structure formation in the universe; and more detailed studies of supposed astrophysical evidence like the Bullet Cluster, have all tended to reinforce the conclusion for me that dark matter probably does exist and is an as-yet undetected particle or class of particles.

I'm still a bit heterodox in my view about it though in that I hold out the hope for a significant role for self-interacting dark matter and dark sector forces, because I still think that's potentially the most interesting thing about it.