Dark Matter: The Mystery Substance Physics Still Can’t Identify That Makes Up The Majority Of Our Universe

The ConversationImage credit: ESA/NASA/JPL-Caltech, CC BY


Guest Author Dan Hooper, University of Chicago

The past few decades have ushered in an amazing era in the science of cosmology. A diverse array of high-precision measurements has allowed us to reconstruct our universe’s history in remarkable detail.

And when we compare different measurements – of the expansion rate of the universe, the patterns of light released in the formation of the first atoms, the distributions in space of galaxies and galaxy clusters and the abundances of various chemical species – we find that they all tell the same story, and all support the same series of events.

This line of research has, frankly, been more successful than I think we had any right to have hoped. We know more about the origin and history of our universe today than almost anyone a few decades ago would have guessed that we would learn in such a short time.

But despite these very considerable successes, there remains much more to be learned. And in some ways, the discoveries made in recent decades have raised as many new questions as they have answered.

One of the most vexing gets at the heart of what our universe is actually made of. Cosmological observations have determined the average density of matter in our universe to very high precision. But this density turns out to be much greater than can be accounted for with ordinary atoms.

After decades of measurements and debate, we are now confident that the overwhelming majority of our universe’s matter – about 84 percent – is not made up of atoms, or of any other known substance. Although we can feel the gravitational pull of this other matter, and clearly tell that it’s there, we simply do not know what it is. This mysterious stuff is invisible, or at least nearly so. For lack of a better name, we call it “dark matter.” But naming something is very different from understanding it.

Astronomers map dark matter indirectly, via its gravitational pull on other objects.
NASA, ESA, and D. Coe (NASA JPL/Caltech and STScI), CC BY

For almost as long as we’ve known that dark matter exists, physicists and astronomers have been devising ways to try to learn what it’s made of. They’ve built ultra-sensitive detectors, deployed in deep underground mines, in an effort to measure the gentle impacts of individual dark matter particles colliding with atoms.

They’ve built exotic telescopes – sensitive not to optical light but to less familiar gamma rays, cosmic rays and neutrinos – to search for the high-energy radiation that is thought to be generated through the interactions of dark matter particles.

And we have searched for signs of dark matter using incredible machines which accelerate beams of particles – typically protons or electrons – up to the highest speeds possible, and then smash them into one another in an effort to convert their energy into matter. The idea is these collisions could create new and exotic substances, perhaps including the kinds of particles that make up the dark matter of our universe.

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