What is Dark Matter?

As of right now, nobody knows what dark matter is because it's completely invisible. Scientists have more knowledge on what dark matter is not, than on what it is. NASA has a list of several things that dark matter is not, including the visible matter like stars and planets; it is not normal matter, made up of baryons, which can be detected by absorption of radiation; it is not antimatter, which can be detected by the unique gamma rays produced by antimatter's annihilation of normal matter; finally, it's not galaxy-sized black holes, because of the amount of gravitational lenses that can be seen.

However, scientists have some theories as to what dark matter could be.

One of the popular hypotheses going around in scientific circles is that dark matter is "made up of exotic particles that don't interact with normal matter or light, but still exert a gravitational pull" (National Geographic Science). One group working within this theory is located a CERN's Large Hadron Collider, trying to find evidence of dark matter. The collider has been under construction and repair for the past two years, and only a couple of weeks ago was restarted again. With the new repairs to the collider, scientists are hoping to create dark matter particles with the Large Hadron Collider. Another, less popular, theory is that baryons could be dark matter, but only under certain terms. NASA postulates that baryons could be dark matter if it were "all tied up in brown dwarfs, or in small, dense chunks of heavy elements."

One of the significant properties of dark matter is that it plays a big part in the expansion of the universe. As we get to the outer reaches of the visible universe, the galaxies and other visible matter seems to be moving faster than those closer to us. It has been determined that dark matter and energy play a large part of this, speeding up the expansion. This is one of the ways that we know dark matter exists, despite being invisible.

(This photo shows the Big Bang, through to the modern universe. You'll notice that it gets bigger as it moves to the right, showing the expansion of the known universe.)

Sources:

http://science.nationalgeographic.com/science/space/dark-matter/

http://www.bbc.com/news/science-environment-32303622

http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/

http://fc04.deviantart.net/fs10/i/2006/116/c/3/Universe_Timeline_by_rblee83.jpg