Concerning the total energy and mass in the universe, about 73% is dark energy. Of the remaining 27%, about 4% is normal matter - all of periodic table, gas, dust, the planets and stars. Most of it is hydrogen. The rest, that is 23%, is dark matter. It's called dark as it doesn't emit or absorb electromagnetic radiation. It doesn't shine nor cast a shadow. It is called matter because it has mass. The only way we know it's about is because it interacts through gravity.
The first observation was made by Fritz Zwicky. He was looking at the speed of rotation of orbits. Planets close to the sun feel a strong pull of gravity so Mercury is zipping around but Neptune doesn't feel such a pull. You'd expect there to be a maximum speed and then to slow down but it reaches a maximum level and stays there so there must be more mass than we can see. It causes distortions which can be detected by Hubble and mapped. This shows galaxies embedded in dark matter.
There are alternative theories to explain the apparent gravity problems which do not require the existence of dark matter.
http://www.sixtysymbols.com/videos/darkmatter.htm
http://www.youtube.com/watch?v=iDRqfX4L2tI&
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