Beautiful as it is, the Universe hides a great deal of its secrets from us. All the stars, galaxies, and other objects we see mask the presence of another substance that comprises around 84% of all the mass in the cosmos. That substance, which in our ignorance we call dark matter, has proven annoyingly elusive for the better part of a century.
Detecting dark matter directly has proved to be tricky, but there is strong evidence for its existence, and it comes from a variety of sources. The first hint of dark matter's existence came in 1933, when Swiss astronomer Fritz Zwicky calculated that the Virgo Cluster of galaxies didn't have enough visible matter in the form of gas and stars to hold it together. It must be embedded in a halo of something invisible, or else it would fly apart. In the early 1970s, American astronomer Vera Rubin demonstrated the magnitude of the missing mass problem when she showed that the outer regions of spiral galaxies rotated far too quickly unless the galaxies contained a lot more mass than could be seen.
In the intervening decades, the evidence for dark matter has only grown stronger. Galaxy clusters—including the iconic "Bullet Cluster" discovered in 2006 – clearly contain far more mass than is visible in stars and gas. On even larger scales, galaxies collect into clumps and long filaments, rather than falling into random configurations, and surround regions largely devoid of matter.
So if dark matter does exist, what is it made of? Right now, a far easier question to answer is what dark matter isn't. First of all, the name is misleading: dark matter isn't "dark" in any usual sense of the word. "Invisible matter" is a better term: light shining on dark matter from any source passes right through without being absorbed or scattered, regardless of the type of light. This means dark matter can't be made of atoms or of their constituent parts; that is, electrons, protons and neutrons.
In fact, dark matter doesn't correspond to anything in the Standard Model, the best explanation we have for how the universe works. The Standard Model describes many aspects of ordinary matter as we know it, along with three of the four fundamental forces: the electromagnetic force, the weak force, and the strong force. (The fourth force, gravitation, lies beyond the Standard Model, it is governed by Einstein’s general theory of relativity.) It also includes a lot of objects that exist ephemerally –including particles made of exotic quarks that decay very quickly – or which are common but hard to detect, such as neutrinos. The final piece of the Standard Model may have been found last summer, with the discovery of something resembling the Higgs boson at Cern.
Dark matter is none of the above. In particle physics terms, dark matter doesn't interact with the electromagnetic force, which governs light, and it ignores the strong force, which binds atomic nuclei together. But understanding how it behaves gravitationally does offer some clues to its make-up.
The way in which galaxies collect into clumps and long filaments makes most sense if the Universe began as a nearly uniform cloud of dark matter, but with tiny fluctuations in density, where a little more dark matter than average would sit. That over-density would attract more matter, creating lumps in some places and emptying out other regions. And thanks to people measuring the faint echoes of the birth of the universe – the cosmic microwave background – we know how much matter is made of atoms, and how much is invisible.