Dark matter is an enduring and exciting puzzle in physics. Multiple astrophysical observations give really strong evidence that most of the matter out there in the universe is invisible – or, more precisely, only affects other matter via its gravitational attraction. Cosmological models, which we use to understand (and simulate) how the universe developed over time, also require dark matter in order to describe what we see around us. There is always the possibility that we don’t understand gravity and cosmology as well as we think we do, but it is very hard to modify gravity in a way that describes all the data. So many tweaks that need to be made, and special cases accounted for, that most physicists give up and reluctantly accept the idea that 85% of the matter in the universe is dark. Worse, while neutrinos are invisible enough to fit the bill, they are not massive enough, and therefore they don’t clump together enough, so can only account for a tiny fraction of the total dark matter. All the other particles in the Standard Model of particle physics interact with light. So they aren’t dark. So dark matter has to be something very new, which makes it interesting for particle physics as well as cosmology. There are at least three very different ways of looking for dark matter. You can try and make it in particle accelerators. You can look for signs of dark matter particles annihilating each other in the centre of massive bodies that they must clump around – the centres of galaxies, maybe even the sun. And perhaps most directly, you can look for rare, glancing collisions between normal matter in a detector here on Earth, and the dark matter cloud the Earth is assumed to be continually rushing through. On the accelerator front, we have had no luck so far, but watch out when the Large Hadron Collider at CERN steps up in energy next year. Lately though, experiments using the other two methods have seen a variety of often-contradictory hints. This is a sign of how much progress is being made in improved sensitivity, but also of how challenging the search is. Not all of these hints can be right. via theguardian.com.
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