What Is Dark Matter Made of?

As far back as the 1930s, evidence for the existence of a “dark matter” in the universe began to emerge. Swiss astronomer Fritz Zwicky measured the velocities of several galaxies in the Coma cluster, a group of more than 1,000 identified galaxies, and concluded that many of them were moving so fast that they should have escaped the gravitational pull of the other galaxies. Zwicky, and other astronomers noticing the same phenomenon, concluded “that something we have yet to detect is providing these galaxies with additional mass, which generates the extra gravity they need to stay intact. This “something” is invisible— hence the nickname “dark matter.” But exactly what is dark matter, and what is it made of?

NASA notes that we’re “more certain what dark matter is not than we are what it is.” Dark matter does not take the form of stars and planets we can see, yet it constitutes about 27 percent of all the matter in the universe. It is not made of baryonic matter, the protons, electrons, and neutrons that make up regular space matter such as stars, planets, rocks, and gas clouds. It does not absorb, emit, or reflect light—the very reason it is extremely difficult to see. In fact, we can only infer its existence based on its gravitational effects on the motions of galaxies and stars.

So what is dark matter made of? The most common view is that dark matter is composed of weakly interacting massive particles, or WIMPS. These particles interact weakly with baryonic matter via gravity. WIMPS have as much as 100 times the mass of a proton, but their weak interactions with baryonic matter make them nearly impossible to see. Other nonbaryonic candidates include neutralinos, hypothetical heavy particles; the smaller neutrinos, subatomic particles without charge; and photinos, a hypothetical subatomic particle. Some scientists believe that dark matter may be composed of bodies of baryonic matter that emit little light and drift through space unattached to any single solar system. Because they emit no light, these bodies, called massive compact halo objects, or MACHOs, would be difficult to detect. A clearer understanding of the composition of dark matter could help scientists better understand the nature of Our universe—especially, how galaxies hold together.

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