We don't know what dark matter is. That is the very strange thing about it! We can detect it only by its gravitational effect on other objects in space. For example: dark matter seems to be present in the halos of galaxies, causing the rotation rates on the outer edges of the disks to be constant instead of decreasing, as we observe for the outer planets in the solar system. It appears to be in between galaxies, and acts as a gravitational lens, distorting the images of distant galaxies similar to the way things look distorted if you look through a wine glass which isfilled with water. Dark matter does not interact with normal matter, whichmakes it really hard to find! But we think that dark matter particles arestreaming through the earth all the time! People are trying to find it by putting detectors deep in mines. If a dark matter particle hits a proton,there should be a transfer of momentum which should temporarily cause the proton to spin differently. So far, though, none have been conclusively detected./p>

Here's a good website to start reading more about dark matter:
Click_here
This is written by Martin White, a professor at UC Berkeley, and an expert in this subject.

Dark matter is a type of matter that scientists think exists in the universe, but aren't totally sure yet. The reason why it's called "dark" matter is because no one has seen dark matter yet - we're pretty unsure what it is. Astronomers have many ideas for what dark matter could be, and many of those ideas think dark matter could be a new form of matter, unlike electrons, protons, and neutrons. However, these hypotheses are unproven at this point.

So how to scientists know that dark matter exists? When astronomers look out into the universe, they can see the affect matter has on other matter by looking at how they interact through gravity. When looking at many places in the universe, scientists see the effects of gravity, but can't "see" all of the actual matter that is causing the gravitational effects. This is dark matter. As far as we can tell right now, dark matter only interacts with other matter through gravity.