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This Is The Real Reason We Haven’t Directly Detected Dark Matter

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 sciencescience in space of space science and universe science

The particles and antiparticles of the Standard Model of particle physics are exactly in line with what experiments require, with only massive neutrinos providing a difficulty and requiring beyond-the-standard-model physics. Dark matter, whatever it is, cannot be any one of these particles, nor can it be a composite of these particles. (E. SIEGEL / BEYOND THE GALAXY) 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 sciencedoble science in space of space science and universe science

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The two bright, large galaxies at the center of the Coma Cluster, NGC 4889 (left) and the slightly smaller NGC 4874 (right), each exceed a million light years in size. But the galaxies on the outskirts, zipping around so rapidly, points to the existence of a large halo of dark matter throughout the entire cluster. (ADAM BLOCK/MOUNT LEMMON SKYCENTER/UNIVERSITY OF ARIZONA)

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 predicted abundances of helium-4, deuterium, helium-3 and lithium-7 as predicted by Big Bang Nucleosynthesis, with observations shown in the red circles. The Universe is 75–76% hydrogen, 24–25% helium, a little bit of deuterium and helium-3, and a trace amount of lithium by mass. After tritium and beryllium decay away, this is what we’re left with, and this remains unchanged until stars form. Only about 1/6th of the Universe’s matter can be in the form of this normal (baryonic, or atom-like) matter. (NASA / WMAP SCIENCE TEAM)

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 were first measured accurately by COBE in the 1990s, then more accurately by WMAP in the 2000s and Planck (above) in the 2010s. This image encodes a huge amount of information about the early Universe, including its composition, age, and history. The fluctuations are only tens to hundreds of microkelvin in magnitude, but definitively point to the existence of both normal and dark matter in a 1:5 ratio. (ESA AND THE PLANCK COLLABORATION)

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.

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