Finding the particle we assume is responsible for dark matter has always been a guessing game. We guessed wrong.
You can’t get mad at a team for trying the improbable, hoping that nature cooperates. Some of the most famous discoveries of all time have come about thanks to nothing more than mere serendipity, and so if we can test something at low-cost with an insanely high reward, we tend to go for it. Believe it or not, that’s the mindset that’s driving the direct searches for dark matter. science in space of space science and universe science
In order to understand how to find dark matter, however, you have to first understand what we know so far, and what the evidence points to as far as direct detection goes. We haven’t found it yet, but that’s okay. Not finding dark matter in an experiment is not evidence that dark matter doesn’t exist. The indirect evidence all shows that it’s real. The question before us is how to demonstrate its reality, hopefully by finding the particle responsible for it directly. science in space of space science and universe science
Let’s begin with a basic recap of dark matter: the idea, the motivation, the observations, the theory and then we’ll talk about the hunt. science in space of space science and universe science
The idea. You know the basics: there are all the protons, neutrons and electrons that make up our bodies, our planet and all the matter we’re familiar with, as well as some photons (light, radiation, etc.) thrown in there for good measure. Protons and neutrons can be broken up into even more fundamental particles — the quarks and gluons — and along with the other Standard Model particles, make up all the known matter in the Universe. science in space of space science and universe science
The big idea of dark matter is that there’s something other than these known particles contributing in a significant way to the total amounts of matter in the Universe. Why would we think such a thing? science in space of space science and universe science
science in space of space science and universe science
The motivation. We know how stars work, and we know how gravity works. If we look at galaxies, clusters of galaxies and go all the way up to the largest-scale structures in the Universe, we can extrapolate two things. One: how much mass there is in these structures at every level. We look at the motions of these objects, we look at the gravitational rules that govern orbiting bodies, whether something is bound or not, how it rotates, how structure forms, etc., and we get a number for how much matter there has to be in there. Two: we know how stars work, so as long as we can measure the starlight coming from these objects, we can know how much mass is there in stars.
These two numbers don’t match, and they don’t match spectacularly. There had to be something more than just stars responsible for the vast majority of mass in the Universe. This is true for the stars within individual galaxies of all sizes all the way up to the largest clusters of thousands of galaxies in the Universe.
The observations. This is where it gets fun, because there are a ton of them; I’ll focus on just three. When we extrapolate the laws of physics all the way back to the earliest times in the Universe, we find that there was not only a time so early when the Universe was hot enough that neutral atoms couldn’t form, but there was a time where even nuclei couldn’t form! The formation of the first elements in the Universe after the Big Bang — due to Big Bang Nucleosynthesis — tells us with very, very small errors how much total “normal matter” is there in the Universe. Although there is significantly more than what’s around in stars, it’s only about one-sixth of the total amount of matter we know is there.
The fluctuations in the cosmic microwave background are particularly interesting. They tell us what fraction of the Universe is in the form of normal (protons+neutrons+electrons) matter, what fraction is in radiation, and what fraction is in non-normal, or dark matter, among other things. Again, they give us that same ratio: that dark matter is about five-sixths of all the matter in the Universe.