Sidebar: Sorting Out the Dark Stuff
With names like "dark matter" and "dark energy", it is easy to confuse the two and to assume that they have some direct relationship to each other. However, dark matter and dark energy are two very different beasts. This article is meant to help students differentiate between the two. Here is a brief description to help:
Dark matter: Dark matter is a type of matter that has been indirectly observed by its gravitational effect on large-scale objects in our Universe, such as galaxies and galaxy clusters. Without the gravitational effect of some additional kind of matter, the galaxies and clusters of galaxies that we see would not be held together. This additional matter is called dark matter, but it has not been directly observed.
Dark energy: Dark energy is a type of energy that has been indirectly observed through its effect on the expansion rate of the Universe. Without dark energy, we would expect that the expansion of the Universe would be slowing down over time, due to the mutual gravity of the matter and energy in the Universe. However, the expansion appears to be speeding up over time, leading astronomers to conclude that there is "something" out there that counteract gravity on the largest scales. That "something" is called dark energy, but it has not be directly observed.
What do they have in common? Both dark matter and dark energy have only been indirectly observed, and both have the word "dark" in their names.
What makes them different? Comparing dark matter with dark energy is a bit like comparing the earth and sunlight. The two are not the same type of object. One is a form of matter that gravitates just like any other matter in the Universe (dark matter). The other is a form of energy that acts differently from any other energy that we have encountered in the Universe (dark energy).
It is important to know that dark matter and dark energy are different, and their presence is seen in different types of observations. Astronomers know certain characteristics of each, but when it comes down to it, they aren't really sure what either one is yet.