NASA/Robert Williams and the Hubble Deep Field Team The Hubble Space Telescope took this "deepest ever" view of the universe. See more astronomy images.

When you look up at night, you see myriads of stars spread across the sky. When astronomers look into the deepest reaches of the universe with powerful telescopes, they see myriads of galaxies, organized into large clusters and other structures. This might lead you to believe that the universe is composed mainly of galaxies, stars, gas and dust -- things that you can see. However, most astronomers believe that visible matter makes up only a small fraction of the mass of the universe. The majority of the universe is made of stuff we can't see -- so-called dark matter. Exactly what is dark matter? How can we detect it? What is its importance in the universe as a whole?

In this article, we'll examine these questions. We will look at the evidence for dark matter, how it can be detected and studied, the nature of dark matter, and how it helps define the structure and fate of the universe.

What is Dark Matter?
Simply put, dark matter cannot be seen by astronomers with telescopes. It doesn't emit or reflect enough light to detect, so it's not bright, like a star. Atoms, molecules and subatomic particles are dark matter. You and I are dark matter. Everything on Earth is dark matter. Planets, brown dwarf stars and black holes are dark matter. Basically, dark matter cannot be seen -- scientists can only estimate where it is based on gravitational effects on what they can see.

We can't see dark matter, but we can detect it by its effects on normal matter through gravity (rotation, gravitational-lensing) and by the X-rays emitted by hot, dark matter. So, what actually is dark matter? What is it made of? Let's take a look.

Video Gallery: Dark Matter University of Chicago Professor Michael Turner clarifies how theoretical and experimental cosmologists challenge each other to unravel the deep mysteries of the universe.

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