Dark Matter

You’ve probably heard the term “dark matter,” but what does it mean? In short, it is a name given to matter that gives off so little light — or no light at all — that we cannot see it even with powerful telescopes. In other words, the term “dark” is used as a contrast to matter that gives off light that we can see. This idea will make more sense if we explore how scientists have concluded that dark matter must really exist.

Scientists can in general use two different methods to learn about a galaxy’s mass.

• The first method is to look at how bright the galaxy is. Because a galaxy’s light is essentially the combined light of all its stars, scientists can use the brightness of a galaxy to estimate the total number of stars in the galaxy. Scientists know the average mass of stars, and therefore can use this number to estimate the mass of the galaxy that is contained in stars.
• The second method is to carefully study of the speeds at which stars (and gas clouds) orbit the center of the galaxy. Because stars are held in orbit by gravity, and because the strength of gravity depends on mass, scientists can use the speeds of stars to calculate how much mass the galaxy contains.

If a galaxy were made mostly of stars — which we might call “light-giving matter” since stars emit light — then both methods would give similar answers for the galaxy’s mass. However, scientists have found that the second method actually gives a much larger mass (typically 10 times as large). The only way that a galaxy’s total mass can be so much larger than the mass of its stars is if most of the matter doesn’t give off light; that is, most of the matter must be dark matter.

Note that, despite the fact that we have given this matter a name (“dark matter”), no one yet knows exactly what it is. There is even a small chance that it does not exist at all, and that the calculations indicating its existence are instead indicating that we don’t have as complete an understanding of gravity as most scientists think we do. Either way, the existence and nature of dark matter is one of the greatest mysteries in science today. Perhaps you will someday help us solve this mystery.